Kurt Lang has one solution.  Another one would be to try a utility like Onyx/Snow Leopard Cache Cleaner.  His solution is the safest however and probably faster as well.  ”
Usually an indication that Font Book’s database and/or collections are damaged. You can reset it by restarting the Mac in Safe Mode (restart and immediately hold down the shift key), then restart normally. This will clear Font Book’s database, font collections and all cache data from your user account.

Be aware that all fonts will be active regardless of their state beforehand, and all collections you’ve created will be gone. Unfortunately, there isn’t another way to fix this type of problem.”  This fixed the problem for the poster.