It seems that the bug only affects those who had a guest account already enabled on their computer before they upgraded to Snow Leopard. Basically, it treats main accounts as guest ones, meaning that when you log out of your main account, bye-bye to any data you’d saved on there.
Those who have logged into a guest account and then back into their main account have found the latter reset to factory defaults, with all personal data and files vanished into the ether.
If you use a backup service such as Time Machine, the data can be retrieved, but if not it seems that it’s gone forever.
Apple has released a statement saying that the problem “only occurs in extremely rare cases” and it’s beavering away on a fix. In the meantime, users should delete any old guest accounts and create new ones that are “native” to Snow Leopard.