It appears that some users of Mac OS X 10.6 Snow Leopard are losing all of their data after logging into a guest account.
Looks like nobody's been able to reproduce this reliably; it doesn't always happen. But it's happening enough that it seems like a good idea to disable the guest account on your computer. At least one person accidentally clicked their guest account, logged out, logged in to their normal account, and found all their data gone.
Of course, it's not certain that the problem is connected to guest accounts. So take this as a reminder to go back up all your data.