Have you upgraded to Snow Leopard? Users on Apple’s forums are reporting a bug that deletes all user data if someone mistakenly logs into the “Guest” account. This issue was originally reported about a month ago on Cnet’s MacFixIt but today is being mentioned across the web as more people are being bitten by the bug.
While not completely reproducible, it is happening frequently enough to take a basic action: turn off the guest account.
eHow explains how to turn off the guest account – it’s quick and takes just a minute.
Of course, you should also make sure your data is backed up, but I assume you already use Backblaze to do this.