Media event set for September 9, launch likely ~10 days later.
Snow Leopard Bug Responsible for Loss of User Data Gaining Notice [Updated]
Further investigation revealed initial reports of the problem in early September, soon after Snow Leopard's launch.
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.
Several threads on the issue have popped up on Apple's support forums, but Apple has yet to make an official acknowledgement of the problem.
Update: CNET reports that Apple has acknowledged the issue and is working on a solution.
"We are aware of the issue, which occurs only in extremely rare cases, and we are working on a fix," an Apple representative said in a prepared statement Monday.
It's the first time Apple has said it is looking into the issue.