It appears that Apple’s Snow Leopard has gotten itself an appetite for your data under certain conditions. The issue; which cnet wrote about back on September 19, has to do with how Snow Leopard handles guest accounts.
What happens according to all accounts so far is that shortly before the bug appears a Guest folder appears in the /Users directory. Interestingly enough previous to this point no such folder existed. Now when the computer is restarted you are somehow logged in as a guest and all your files stored under you main account get deleted.
As Ed Oswald at Technologizer notes the only way to recover from this requires that you have a full backup available.
Moral of the story here? If you’re on Snow Leopard, start backing up your home folder regularly.
Apple is currently looking into the issue, however has not provided any guidance as to when the problem could be fixed.