Same for me... have had this iMac about 3 months and it's never crashed. Since installing Lion it crashes maybe once a day and always when I'm playing video in Safari. I'm not 100% sure but it seems to only happen when I'm in full screen mode.
Also noticed BBC iplayer doing quirky things with video, frames not streaming while audio does, or audio being out of sync. Again, this has only happened since Lion update.
Hope there's a patch soon this bites and i'm disappointed as it's been so stable otherwise.
After upgrading to Lion, my Safari refuses to start. Crashes as soon as I try to launch it...
Process: Safari 
Version: 5.1 (7534.48.3)
Build Info: WebBrowser-7534048003000000~1
Code Type: X86-64 (Native)
Parent Process: launchd 
PlugIn Path: /Library/Application Support/SIMBL/*/ZZafari.bundle/Contents/MacOS/ZZafari
PlugIn Identifier: com.cocoamug.zzafari
PlugIn Version: 1.0 (1)
Date/Time: 2011-08-08 00:13:43.838 +0800
OS Version: Mac OS X 10.7 (11A511)
Report Version: 9
Interval Since Last Report: 87550 sec
Crashes Since Last Report: 6
Per-App Interval Since Last Report: 29 sec
Per-App Crashes Since Last Report: 6
Anonymous UUID: 48F2BBAF-83A1-428F-917B-76F5D6CAC46B
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Application Specific Information:
objc: garbage collection is OFF
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[BrowserWKView expectedOrCurrentURL]: unrecognized selector sent to instance 0x7fe94b608a40'
*** First throw call stack:
I posted this problem elsewhere and whilst the Flash beta made it crash less it still crashed..... I found that it was my Intego [Net Barrier etc] software that was the problem. Apparently any anti-virus software you might have does not play well with Lion at all.... since I uninstalled it [it has to be done by the proper uninstaller] I've been fine.
For what it's worth, I had what I think is the same issue after upgrading my iMac to Lion - Firefox/Chrome performed faultlessly but everything would hang as soon as any video was started in Safari thus requiring a power cycle.
Couldn't fathom what whas causing this, but recently bought an Air (Lion preinstalled) and this didn't suffer the problem. Looking at the differences, the only thing I could spot was that I had installed Flip4Mac on the iMac under Snow Leopard. I'd forgotten all about it, so hadn't crossed my mind it might be the cause.
Looking at their website, they still don't support Lion, so anyhow - long story short, I uninstalled F4M, rebooted for good measure and hey presto!
It may not be the exact cause of your problems, but like ~Bee points out, it certainly points to plug-ins
having the same issue.
Safari 5.1 keeps Freezing/Stalling... even when I try clearing the cache.. Reseting Safari etc..
I'm going to try and uninstall Plugins..
I'm only using 2 - YoutubeWiden and AdBlock..
Hopefully there'll be an update to fix this soon though..
I'm new to Mac and do not wish to do any Restoring (Wipe out everything with the disc and reinstall MacSoftware) etc..