Users in this Apple Support Discussions forum were stumped, trying several fixes including deleting plist files, repairing permissions, and deleting cache files, all to no avail. In fact, the issue seems to be somewhat random, appearing on some domain files, but not others (running on the same system). New user accounts in Snow Leopard retained the issue, though transferring the iWeb Domain file (username > Library > Applications Support > iWeb) to a system running 10.5.8 Leopard seemed to work fine.
Apple Support Discussions user "alexdennis" finds a solution after several other users report disabling the Hoefler Text font using Font Book provided a temporary workaround to the problem:
1. Delete the com.apple.iWeb.plist file in Home/Library/Preferences
2. Shut down
3. Boot up holding down the shift key (safe reboot)
4. Shut down again
5. Boot up and run Font Book to resolve duplicated fonts (I've got MS Office 2008 -- you might not need to do this)
6. Shut down and reboot
7. Run iWeb
Many users report that this "total" solution has solved the iWeb issue. ASD user "macuserhelp" reports:
I had disabled the "problem font", and that was a work around that required quite a bit of messing with the site. This however, seems to have fixed the problem, allowing me to use the original font. I had re-enabled the problem font before following your process.