My experience is the same as Mr. Dunham, on the other thread, so please check for System 9 fonts:
http://discussions.apple.com/thread.jspa?threadID=1207302&tstart=0
I too upgraded to Leopard from Tiger, under which I had been using 'Classic Mode' to run old applications, and after upgrading Pages refused to find normal Helvetica. The old applications needed System 9 fonts, so there were lots of them stored on my hard disc. I tried 'archive and re-installing' Leopard, but the problem remained. Only after I had removed ALL the System 9 fonts did the problem disappear, and the system and Pages '08 now find the correct Helvetica. The last font I found was 'Sans Serif', yet this still prevented correct operation. Once thrown into the trash and emptied out (and a re-boot) everything operated flawlessly -- thank God!
It seems that any font in a font folder, which is itself enclosed within an old System 9 'System Folder' gets picked up and installed by Leopard at start-up, even if you haven't installed it yourself, and even if the folder is buried many folders deep. The last ones I found on my hard disc were hidden in a folder containing back-ups of old Zip discs that I had made years ago, that in turn were made to back up the whole of a System 9 hard disc. For some reason Spotlight did not find the Helvetica font file, so I had to search for another font by name, before Spotlight finally found the offending folder of fonts (I suggest searching for 'Sans Serif').
Good luck!.