CNet is reporting that Apple is now aware, and preparing a fix for a bug in Snow Leopard that wipes user data. This bug appears to affect only Guest accounts, in particular when Guest accounts were in enabled under machines running Leopard, but were later upgraded to Snow Leopard. Users would be logged into Guest accounts upon booting their machines, when switching back to their regular account, the user finds that their data has been lost, which is only recoverable from backup.
Apparently this bug has been know for some time now, but has only recently gained attention from Apple.
This could be due to a bug in how guest accounts are managed since data and settings are deleted from these accounts upon logout, but also could be from some corruption in the guest account. So far, it does not seem to be a widespread problem.
If this happens to you, immediately restore using your latest Time Machine backup (or other full system backup), and then go to the Accounts system preferences and disable log-in on the guest account.
One way to avoid this problem is to disable Guest accounts under Leopard, upgrade to Snow Leopard, and then reenabled after the upgrade is complete.
Okay, everyone get out their stopwatches; we’ll see how long it takes Apple to fix this. Not long I wager, if for no other reason than it is a serious bug.