happens when your font caches get corrupted. are you using suitcase or another font management program? if you are, open it up and deactivate some fonts -- if you've got any helveticas open, start there. for some reason, they tend to be the culprit. often, just deactivating fonts solves the problem right away.
if you're not using any font management, or if that doesn't help, try Panther Cache Cleaner (www.northernSW.com) or another 10.3-ready cache cleaning utility.
and good luck... this bug bites me all the freaking time and i'm pretty sick of it.