There might be another problem related to font managers. Check your console log for the following:
7/23/11 11:21:13.385 PM sandboxd: ([5248]) WebProcess(5248) deny file-read-data /Users/robert/Documents/Fonts/FontExplorer X/Font Library/C/Calibri 3/Calibri.ttf
If you manage the system fonts in the font manager, then the Safari sandbox would prevent it from reading the font file. Firefox and most other apps aren't sandboxed, which would explain why those would work.
Here's a list of the fonts that Lion puts in /System/Library/Fonts:
-rw-r--r-- 1 root wheel 263912 Jul 20 20:54 Apple Braille Outline 6 Dot.ttf
-rw-r--r-- 1 root wheel 280980 Jul 20 20:54 Apple Braille Outline 8 Dot.ttf
-rw-r--r-- 1 root wheel 183080 Jul 20 20:54 Apple Braille Pinpoint 6 Dot.ttf
-rw-r--r-- 1 root wheel 189668 Jul 20 20:54 Apple Braille Pinpoint 8 Dot.ttf
-rw-r--r-- 1 root wheel 135960 Jul 20 20:54 Apple Braille.ttf
-rw-r--r-- 1 root wheel 20094616 Jun 18 04:04 Apple Color Emoji.ttf
-rw-r--r-- 1 root wheel 3743560 Jul 20 20:54 Apple Symbols.ttf
-rw-r--r-- 1 root wheel 15600596 Jul 20 20:55 AppleGothic.ttf
-rw-r--r-- 1 root wheel 18600216 Jul 20 20:55 AquaKana.ttc
-rw-r--r-- 1 root wheel 1342469 Jul 20 20:54 Courier.dfont
-rw-r--r-- 1 root wheel 272984 Jul 20 20:54 Geeza Pro Bold.ttf
-rw-r--r-- 1 root wheel 271732 Jul 20 20:54 Geeza Pro.ttf
-rw-r--r-- 1 root wheel 856010 Jul 20 20:54 Geneva.dfont
-rw-r--r--@ 1 root wheel 0 Jun 18 04:03 HelveLTMM
-rw-r--r--@ 1 root wheel 0 Jun 18 04:03 Helvetica LT MM
-rw-r--r-- 1 root wheel 2919210 Jul 20 20:54 Helvetica.dfont
-rw-r--r-- 1 root wheel 3536121 Jul 20 20:54 HelveticaNeue.dfont
-rw-r--r-- 1 root wheel 1576420 Jul 20 20:54 HelveticaNeueDeskUI.ttc
-rw-r--r-- 1 root wheel 28564 Jul 20 20:54 Keyboard.ttf
-rw-r--r-- 1 root wheel 13894924 Jul 20 20:54 LastResort.ttf
-rw-r--r-- 1 root wheel 1766684 Jul 20 20:54 LucidaGrande.ttc
-rw-r--r-- 1 root wheel 650364 Jul 20 20:54 MarkerFelt.ttc
-rw-r--r-- 1 root wheel 2172912 Jul 20 20:54 Menlo.ttc
-rw-r--r-- 1 root wheel 567939 Jul 20 20:54 Monaco.dfont
-rw-r--r-- 1 root wheel 56703440 Jun 13 14:08 STHeiti Light.ttc
-rw-r--r-- 1 root wheel 56545420 Jun 13 14:08 STHeiti Medium.ttc
-rw-r--r-- 1 root wheel 103400 Jul 20 20:54 Symbol.ttf
-rw-r--r-- 1 root wheel 224784 Jul 20 20:55 Thonburi.ttf
-rw-r--r-- 1 root wheel 256856 Jul 20 20:55 ThonburiBold.ttf
-rw-r--r--@ 1 root wheel 0 Jun 18 04:03 Times LT MM
-rw-r--r-- 1 root wheel 1743667 Jul 20 20:54 Times.dfont
-rw-r--r--@ 1 root wheel 0 Jun 18 04:03 TimesLTMM
-rw-r--r-- 1 root wheel 154132 Jul 20 20:54 ZapfDingbats.ttf
-rw-r--r-- 1 root wheel 9699132 Jul 20 20:55 ヒラギノ明朝 ProN W3.otf
-rw-r--r-- 1 root wheel 9810576 Jul 20 20:55 ヒラギノ明朝 ProN W6.otf
-rw-r--r-- 1 root wheel 7476024 Jul 20 20:55 ヒラギノ角ゴ ProN W3.otf
-rw-r--r-- 1 root wheel 7523672 Jul 20 20:55 ヒラギノ角ゴ ProN W6.otf
and these are in /Library/Fonts:
-rw-r--r-- 1 root wheel 82432 Jul 20 20:55 AlBayan.ttf
-rw-r--r-- 1 root wheel 82200 Jul 20 20:55 AlBayanBold.ttf
-rw-r--r-- 1 root wheel 923708 Jul 20 20:54 AmericanTypewriter.ttc
-rw-r--r-- 1 root wheel 109700 Jul 20 20:54 Andale Mono.ttf
-rw-r--r-- 1 root wheel 336776 Jul 20 20:54 Apple Chancery.ttf
-rw-r--r-- 1 root wheel 5802828 Jul 20 20:54 Apple LiGothic Medium.ttf
-rw-r--r-- 1 root wheel 8881092 Jul 20 20:54 Apple LiSung Light.ttf
-rw-r--r-- 1 root wheel 19108764 Jul 20 20:55 AppleMyungjo.ttf
-rw-r--r-- 1 root wheel 122556 Jul 20 20:54 Arial Black.ttf
-rw-r--r-- 1 root wheel 558672 Jul 20 20:54 Arial Bold Italic.ttf
-rw-r--r-- 1 root wheel 750984 Jul 20 20:54 Arial Bold.ttf
-rw-r--r-- 1 root wheel 553284 Jul 20 20:54 Arial Italic.ttf
-rw-r--r-- 1 root wheel 183932 Jul 20 20:54 Arial Narrow Bold Italic.ttf
-rw-r--r-- 1 root wheel 184420 Jul 20 20:54 Arial Narrow Bold.ttf
-rw-r--r-- 1 root wheel 184944 Jul 20 20:54 Arial Narrow Italic.ttf
-rw-r--r-- 1 root wheel 179492 Jul 20 20:54 Arial Narrow.ttf
-rw-r--r-- 1 root wheel 49296 Jul 20 20:54 Arial Rounded Bold.ttf
-rw-r--r-- 1 root wheel 23278008 Jul 20 20:54 Arial Unicode.ttf
-rw-r--r-- 1 root wheel 773236 Jul 20 20:54 Arial.ttf
-rw-r--r-- 1 root wheel 36216 Jul 20 20:55 ArialHB.ttf
-rw-r--r-- 1 root wheel 35548 Jul 20 20:55 ArialHBBold.ttf
-rw-r--r-- 1 root wheel 295396 Jul 20 20:55 Ayuthaya.ttf
-rw-r--r-- 1 root wheel 99492 Jul 20 20:55 Baghdad.ttf
-rw-r--r-- 1 root wheel 840072 Jul 20 20:55 Bangla MN.ttc
-rw-r--r-- 1 root wheel 386664 Jul 20 20:55 Bangla Sangam MN.ttf
-rw-r--r-- 1 root wheel 1130252 Jul 20 20:54 Baskerville.ttc
-rw-r--r-- 1 root wheel 13911192 Jul 20 20:54 BiauKai.ttf
-rw-r--r-- 1 root wheel 140976 Jul 20 20:54 BigCaslon.ttf
-rw-r--r-- 1 root wheel 56844 Jul 20 20:54 Brush Script.ttf
-rw-r--r-- 1 root wheel 184212 Jul 20 20:54 Chalkboard.ttc
-rw-r--r-- 1 root wheel 483652 Jul 20 20:54 Chalkduster.ttf
-rw-r--r-- 1 root wheel 68268 Jul 20 20:55 CharcoalCY.dfont
-rw-r--r-- 1 root wheel 1223332 Jul 20 20:54 Cochin.ttc
-rw-r--r-- 1 root wheel 120120 Jul 20 20:54 Comic Sans MS Bold.ttf
-rw-r--r-- 1 root wheel 135484 Jul 20 20:54 Comic Sans MS.ttf
-rw-r--r-- 1 root wheel 331884 Jul 20 20:54 Copperplate.ttc
-rw-r--r-- 1 root wheel 40340 Jul 20 20:55 Corsiva.ttf
-rw-r--r-- 1 root wheel 42096 Jul 20 20:55 CorsivaBold.ttf
-rw-r--r-- 1 root wheel 506640 Jul 20 20:54 Courier New Bold Italic.ttf
-rw-r--r-- 1 root wheel 691796 Jul 20 20:54 Courier New Bold.ttf
-rw-r--r-- 1 root wheel 589900 Jul 20 20:54 Courier New Italic.ttf
-rw-r--r-- 1 root wheel 684624 Jul 20 20:54 Courier New.ttf
-rw-r--r-- 1 root wheel 516276 Jul 20 20:55 Damascus.ttc
-rw-r--r-- 1 root wheel 67956 Jul 20 20:55 DecoTypeNaskh.ttf
-rw-r--r-- 1 root wheel 823404 Jul 20 20:55 Devanagari Sangam MN.ttc
-rw-r--r-- 1 root wheel 332324 Jul 20 20:55 DevanagariMT.ttf
-rw-r--r-- 1 root wheel 315620 Jul 20 20:55 DevanagariMTBold.ttf
-rw-r--r-- 1 root wheel 352996 Jul 20 20:54 Didot.ttc
-rw-r--r-- 1 root wheel 547808 Jul 20 20:55 EuphemiaCAS.ttc
-rw-r--r-- 1 root wheel 321008 Jul 20 20:54 Futura.ttc
-rw-r--r-- 1 root wheel 108080 Jul 20 20:55 GenevaCY.dfont
-rw-r--r-- 1 root wheel 167476 Jul 20 20:54 Georgia Bold Italic.ttf
-rw-r--r-- 1 root wheel 148768 Jul 20 20:54 Georgia Bold.ttf
-rw-r--r-- 1 root wheel 165208 Jul 20 20:54 Georgia Italic.ttf
-rw-r--r-- 1 root wheel 159656 Jul 20 20:54 Georgia.ttf
-rw-r--r-- 1 root wheel 554220 Jul 20 20:54 GillSans.ttc
-rw-r--r-- 1 root wheel 244088 Jul 20 20:55 Gujarati Sangam MN.ttf
-rw-r--r-- 1 root wheel 224940 Jul 20 20:55 GujaratiMT.ttf
-rw-r--r-- 1 root wheel 213708 Jul 20 20:55 GujaratiMTBold.ttf
-rw-r--r-- 1 root wheel 10658192 Jul 20 20:55 Gungseouche.ttf
-rw-r--r-- 1 root wheel 186456 Jul 20 20:55 Gurmukhi MN.ttc
-rw-r--r-- 1 root wheel 64572 Jul 20 20:55 Gurmukhi.ttf
-rw-r--r-- 1 root wheel 1215336 Jul 20 20:55 HeadlineA.ttf
-rw-r--r-- 1 root wheel 7502752 Jul 20 20:54 Hei.ttf
-rw-r--r-- 1 root wheel 276385 Jul 20 20:55 HelveticaCY.dfont
-rw-r--r-- 1 root wheel 106264 Jul 20 20:54 Herculanum.ttf
-rw-r--r-- 1 root wheel 13178508 Jun 13 14:08 Hiragino Sans GB W3.otf
-rw-r--r-- 1 root wheel 13240508 Jun 13 14:08 Hiragino Sans GB W6.otf
-rw-r--r-- 1 root wheel 40984 Jul 20 20:54 Hoefler Text Ornaments.ttf
-rw-r--r-- 1 root wheel 1640424 Jul 20 20:54 Hoefler Text.ttc
-rw-r--r-- 1 root wheel 138488 Jul 20 20:54 Impact.ttf
-rw-r--r-- 1 root wheel 263572 Jul 20 20:55 InaiMathi.ttf
-rw-r--r-- 1 root wheel 7888604 Jul 20 20:54 Kai.ttf
-rw-r--r-- 1 root wheel 620764 Jul 20 20:55 Kailasa.ttf
-rw-r--r-- 1 root wheel 1284516 Jul 20 20:55 Kannada MN.ttc
-rw-r--r-- 1 root wheel 530320 Jul 20 20:55 Kannada Sangam MN.ttf
-rw-r--r-- 1 root wheel 391476 Jul 20 20:55 Kefa.ttc
-rw-r--r-- 1 root wheel 741648 Jul 20 20:55 Khmer MN.ttc
-rw-r--r-- 1 root wheel 284904 Jul 20 20:55 Khmer Sangam MN.ttf
-rw-r--r-- 1 root wheel 974272 Jul 20 20:55 Kokonor.ttf
-rw-r--r-- 1 root wheel 227948 Jul 20 20:55 Krungthep.ttf
-rw-r--r-- 1 root wheel 79988 Jul 20 20:55 KufiStandardGK.ttf
-rw-r--r-- 1 root wheel 173452 Jul 20 20:55 Lao MN.ttc
-rw-r--r-- 1 root wheel 87788 Jul 20 20:55 Lao Sangam MN.ttf
-rw-r--r-- 1 root wheel 339640 Jul 20 20:55 Malayalam MN.ttc
-rw-r--r-- 1 root wheel 184816 Jul 20 20:55 Malayalam Sangam MN.ttf
-rw-r--r-- 1 root wheel 652248 Jul 20 20:54 Microsoft Sans Serif.ttf
-rw-r--r-- 1 root wheel 37320 Jul 20 20:55 MshtakanBold.ttf
-rw-r--r-- 1 root wheel 41400 Jul 20 20:55 MshtakanBoldOblique.ttf
-rw-r--r-- 1 root wheel 51360 Jul 20 20:55 MshtakanOblique.ttf
-rw-r--r-- 1 root wheel 46172 Jul 20 20:55 MshtakanRegular.ttf
-rw-r--r-- 1 root wheel 447432 Jul 20 20:55 Myanmar MN.ttc
-rw-r--r-- 1 root wheel 202836 Jul 20 20:55 Myanmar Sangam MN.ttf
-rw-r--r-- 1 root wheel 7108232 Jul 20 20:54 NISC18030.ttf
-rw-r--r-- 1 root wheel 59452 Jul 20 20:55 Nadeem.ttf
-rw-r--r-- 1 root wheel 12790388 Jul 20 20:55 NanumGothic.ttc
-rw-r--r-- 1 root wheel 13193888 Jul 20 20:55 NanumMyeongjo.ttc
-rw-r--r-- 1 root wheel 7242476 Jul 20 20:55 NanumScript.ttc
-rw-r--r-- 1 root wheel 43868 Jul 20 20:55 NewPeninimMT.ttf
-rw-r--r-- 1 root wheel 48116 Jul 20 20:55 NewPeninimMTBold.ttf
-rw-r--r-- 1 root wheel 50384 Jul 20 20:55 NewPeninimMTBoldInclined.ttf
-rw-r--r-- 1 root wheel 44520 Jul 20 20:55 NewPeninimMTInclined.ttf
-rw-r--r-- 1 root wheel 516168 Jul 20 20:54 Noteworthy.ttc
-rw-r--r-- 1 root wheel 567004 Jul 20 20:54 Optima.ttc
-rw-r--r-- 1 root wheel 482636 Jul 20 20:55 Oriya MN.ttc
-rw-r--r-- 1 root wheel 273600 Jul 20 20:55 Oriya Sangam MN.ttf
-rw-r--r-- 1 root wheel 3631328 Jul 20 20:55 Osaka.ttf
-rw-r--r-- 1 root wheel 2307144 Jul 20 20:55 OsakaMono.ttf
-rw-r--r-- 1 root wheel 9515016 Jul 20 20:55 PCmyoungjo.ttf
-rw-r--r-- 1 root wheel 2763792 Jul 20 20:55 PTSans.ttc
-rw-r--r-- 1 root wheel 1701880 Jul 20 20:54 Palatino.ttc
-rw-r--r-- 1 root wheel 210844 Jul 20 20:54 Papyrus.ttc
-rw-r--r-- 1 root wheel 4676380 Jul 20 20:55 Pilgiche.ttf
-rw-r--r-- 1 root wheel 72336 Jul 20 20:55 PlantagenetCherokee.ttf
-rw-r--r-- 1 root wheel 32420 Jul 20 20:55 Raanana.ttf
-rw-r--r-- 1 root wheel 31636 Jul 20 20:55 RaananaBold.ttf
-rw-r--r-- 1 root wheel 414322 Jul 20 20:54 STIXGeneral.otf
-rw-r--r-- 1 root wheel 220018 Jul 20 20:54 STIXGeneralBol.otf
-rw-r--r-- 1 root wheel 174020 Jul 20 20:54 STIXGeneralBolIta.otf
-rw-r--r-- 1 root wheel 170988 Jul 20 20:54 STIXGeneralItalic.otf
-rw-r--r-- 1 root wheel 17512 Jul 20 20:54 STIXIntDBol.otf
-rw-r--r-- 1 root wheel 17696 Jul 20 20:54 STIXIntDReg.otf
-rw-r--r-- 1 root wheel 16688 Jul 20 20:54 STIXIntSmBol.otf
-rw-r--r-- 1 root wheel 17456 Jul 20 20:54 STIXIntSmReg.otf
-rw-r--r-- 1 root wheel 16924 Jul 20 20:54 STIXIntUpBol.otf
-rw-r--r-- 1 root wheel 17268 Jul 20 20:54 STIXIntUpDBol.otf
-rw-r--r-- 1 root wheel 17224 Jul 20 20:54 STIXIntUpDReg.otf
-rw-r--r-- 1 root wheel 16976 Jul 20 20:54 STIXIntUpReg.otf
-rw-r--r-- 1 root wheel 17124 Jul 20 20:54 STIXIntUpSmBol.otf
-rw-r--r-- 1 root wheel 17256 Jul 20 20:54 STIXIntUpSmReg.otf
-rw-r--r-- 1 root wheel 55806 Jul 20 20:54 STIXNonUni.otf
-rw-r--r-- 1 root wheel 29378 Jul 20 20:54 STIXNonUniBol.otf
-rw-r--r-- 1 root wheel 39780 Jul 20 20:54 STIXNonUniBolIta.otf
-rw-r--r-- 1 root wheel 44628 Jul 20 20:54 STIXNonUniIta.otf
-rw-r--r-- 1 root wheel 13598 Jul 20 20:54 STIXSizFiveSymReg.otf
-rw-r--r-- 1 root wheel 12454 Jul 20 20:54 STIXSizFourSymBol.otf
-rw-r--r-- 1 root wheel 15794 Jul 20 20:54 STIXSizFourSymReg.otf
-rw-r--r-- 1 root wheel 12738 Jul 20 20:54 STIXSizOneSymBol.otf
-rw-r--r-- 1 root wheel 19076 Jul 20 20:54 STIXSizOneSymReg.otf
-rw-r--r-- 1 root wheel 12466 Jul 20 20:54 STIXSizThreeSymBol.otf
-rw-r--r-- 1 root wheel 15774 Jul 20 20:54 STIXSizThreeSymReg.otf
-rw-r--r-- 1 root wheel 12402 Jul 20 20:54 STIXSizTwoSymBol.otf
-rw-r--r-- 1 root wheel 15656 Jul 20 20:54 STIXSizTwoSymReg.otf
-rw-r--r-- 1 root wheel 22008 Jul 20 20:54 STIXVar.otf
-rw-r--r-- 1 root wheel 14988 Jul 20 20:54 STIXVarBol.otf
-rw-r--r-- 1 root wheel 264804 Jul 20 20:55 Sathu.ttf
-rw-r--r-- 1 root wheel 277524 Jul 20 20:55 Silom.ttf
-rw-r--r-- 1 root wheel 1470148 Jul 20 20:55 Sinhala MN.ttc
-rw-r--r-- 1 root wheel 640764 Jul 20 20:55 Sinhala Sangam MN.ttf
-rw-r--r-- 1 root wheel 475656 Jul 20 20:54 Skia.ttf
-rw-r--r-- 1 root wheel 626928 Jul 20 20:54 Tahoma Bold.ttf
-rw-r--r-- 1 root wheel 681120 Jul 20 20:54 Tahoma.ttf
-rw-r--r-- 1 root wheel 395972 Jul 20 20:55 Tamil MN.ttc
-rw-r--r-- 1 root wheel 171656 Jul 20 20:55 Tamil Sangam MN.ttf
-rw-r--r-- 1 root wheel 1762252 Jul 20 20:55 Telugu MN.ttc
-rw-r--r-- 1 root wheel 644920 Jul 20 20:55 Telugu Sangam MN.ttf
-rw-r--r-- 1 root wheel 620008 Jul 20 20:54 Times New Roman Bold Italic.ttf
-rw-r--r-- 1 root wheel 842168 Jul 20 20:54 Times New Roman Bold.ttf
-rw-r--r-- 1 root wheel 660268 Jul 20 20:54 Times New Roman Italic.ttf
-rw-r--r-- 1 root wheel 834452 Jul 20 20:54 Times New Roman.ttf
-rw-r--r-- 1 root wheel 137008 Jul 20 20:54 Trebuchet MS Bold Italic.ttf
-rw-r--r-- 1 root wheel 129360 Jul 20 20:54 Trebuchet MS Bold.ttf
-rw-r--r-- 1 root wheel 144820 Jul 20 20:54 Trebuchet MS Italic.ttf
-rw-r--r-- 1 root wheel 138848 Jul 20 20:54 Trebuchet MS.ttf
-rw-r--r-- 1 root wheel 173132 Jul 20 20:54 Verdana Bold Italic.ttf
-rw-r--r-- 1 root wheel 153260 Jul 20 20:54 Verdana Bold.ttf
-rw-r--r-- 1 root wheel 174612 Jul 20 20:54 Verdana Italic.ttf
-rw-r--r-- 1 root wheel 186188 Jul 20 20:54 Verdana.ttf
-rw-r--r-- 1 root wheel 124308 Jul 20 20:54 Webdings.ttf
-rw-r--r-- 1 root wheel 68768 Jul 20 20:54 Wingdings 2.ttf
-rw-r--r-- 1 root wheel 38308 Jul 20 20:54 Wingdings 3.ttf
-rw-r--r-- 1 root wheel 86384 Jul 20 20:54 Wingdings.ttf
-rw-r--r-- 1 root wheel 528844 Jul 20 20:54 Zapfino.ttf
-rw-r--r-- 1 root wheel 17965572 Jul 20 20:54 儷宋 Pro.ttf
-rw-r--r-- 1 root wheel 10823828 Jul 20 20:54 儷黑 Pro.ttf
-rw-r--r-- 1 root wheel 15734488 Jul 20 20:54 华文仿宋.ttf
-rw-r--r-- 1 root wheel 16163136 Jul 20 20:54 华文宋体.ttf
-rw-r--r-- 1 root wheel 17502796 Jul 20 20:54 华文楷体.ttf
-rw-r--r-- 1 root wheel 15692256 Jul 20 20:54 华文细黑.ttf
-rw-r--r-- 1 root wheel 13631224 Jul 20 20:54 华文黑体.ttf
-rw-r--r-- 1 root wheel 11580692 Jul 20 20:55 ヒラギノ明朝 Pro W3.otf
-rw-r--r-- 1 root wheel 11092104 Jul 20 20:55 ヒラギノ明朝 Pro W6.otf
-rw-r--r-- 1 root wheel 9362244 Jul 20 20:55 ヒラギノ角ゴ Pro W3.otf
-rw-r--r-- 1 root wheel 10109512 Jul 20 20:55 ヒラギノ丸ゴ Pro W4.otf
-rw-r--r-- 1 root wheel 8871572 Jul 20 20:55 ヒラギノ角ゴ Pro W6.otf
-rw-r--r-- 1 root wheel 9269132 Jul 20 20:55 ヒラギノ丸ゴ ProN W4.otf
-rw-r--r-- 1 root wheel 4276744 Jul 20 20:55 ヒラギノ角ゴ Std W8.otf
-rw-r--r-- 1 root wheel 3639612 Jun 13 19:23 ヒラギノ角ゴ StdN W8.otf
Those may be the only fonts (plus /User/yourname/Library/Fonts) that Safari is allowed to use. Anything less than this, trouble.