A few users have reported in Apple forums incidents of total data loss using Guest account after upgraded to Snow Leopard. Apple has confirmed on Tuesday that the company is “aware of the issue, which only occurs in extremely rare cases, and we are working on a fix.”
According to reports, the bug occurs when you upgrade from Leopard to Snow Leopard with an existing Guest account in Leopard. After the upgrade, all data are gone after users logged into their Guest accounts and then back to their normal accounts.
PC World has unsuccessfully [tried to replicate the bug](http://www.pcworld.com/businesscenter/article/173616/inside_snow_leopards_guest_account_data_loss_bug.html), suggesting it is indeed rare.
It is better be safe than sorry, make sure there is a usable backup before you upgrade to Snow Leopard. And avoid using Guest account in Snow Leopard until Apple fixes the problem.