Computerworld - Snow Leopard users have reported that they've lost all their personal data when they've logged into a "Guest" account after upgrading from Leopard, according to messages on Apple's support forum.
The bug, users said in a well-read thread on Apple's support forum, resets all settings on the Mac, resets all applications' settings and erases the contents of critical folders containing documents, photos and music.
Specifically, Snow Leopard's home directory -- the one sporting the name of the Mac's primary user -- is replaced with a new, empty copy after users log-in to a Guest account, log out, then log-in to their standard account. All the standard folders -- Documents, Downloads, Music, Picture and others -- are empty, while the Desktop and Dock have reverted to an "out-of-box" condition.
Full story here.
The best way to prevent getting bit by this bug is to go into System Preferences > Accounts and disable the Guest account. I also recommend disabling Guest access to shared folders.
Hopefully, Apple will fix this in OS 10.6.2. In the meantime go disable your Guest account if it is still enabled.