I haven't seen much confirmation of this, but I'd be a hypocrite if I didn't point to a story where <screaming alarm voice>a Major bug in Snow Leopard deletes all user data</screaming alarm voice>. It appears to be centered around the guest account, and logging into it, then back into your account deletes all your data (if it's true, Steve Jobs deserves a spanking, cause that's bad), but might also have to do with the way you upgraded to Snow Leopard, upgrade vs fresh install or something. Read the article and watch for news about this, and maybe stop using the guest account until this is all worked out. And maybe switch back to a PC for a bit too....