Handily, by the power of google I found this article which helps explain how to fix the problem, which exists in the OS X Tiger and Leopard (10.4, 10.5).

The basic point here is ‘you have a broken font somewhere.’ I’m not sure what it’s caused by, but I’ll just blame MS Word for Mac 2004, makes me happy so-to-do:
- Go look in your Font Book and whack the duplicate entries (Font Book.app -> Edit -> Resolve Duplicates).
- Next grab a (free) copy of Font Nuke and blast those caches. NB: You’ll magically be restarted, despite the onscreen warning just saying to save documents.

Productive day.