A serious flaw in Apple's Snow Leopard appears capable of wiping user data after the user opens and closes the "guest" account on the afflicted Macintosh.
According to reports, when the user first opens the guest account, closes it, and later logs back into their own account, their Home folder data has been erased.
Although the fault cannot be repeated at will, it has occurred often enough to result in several threads on Apple's online support forum.
Apple has not yet commented on the problem, according to Apple Insider. A simple preventive measure would be to disable the guest account (done in the "Accounts" System Preferences pane).
Users who suffer data loss, and are running Apple's Time Machine backup application, can restore their Home folder from the backup. This is done by pressing and holding 'C' during startup and selecting "Restore from Backup" from the "Utilities" menu.
Introduced in August, Snow Leopard has apparently suffered this problem from the beginning, apparently on machines that already had the guest account enabled before the OS upgrade.
The recent 10.6.1 upgrade did not solve the problem. A fix is hoped for 10.6.2, now in beta testing by developers.
In the meantime: Unless you really, really need that guest account, just turn it off. And make sure Time Machine, or some other backup application, is turned on.