It seems that there is a nasty bug in Snow Leopard for users who had the guest account enabled in Leopard before upgrading. Logging into such a guest account has caused people to lose all the data in their regular accounts.

http://www.pcworld.com/article/173502/upda...le_miss_it.html

The bug is very serious but it doesn't affect every Snow Leopard powered Mac. I've tested many Macs and never had any problem with the guest account & lost user data. That said, I tell our users to disable guest account until this issue is fixed for good.

That guest account was trouble in 10.5 and it has carried up to 10.6. I agree, disable it and don't ever look back!