This Mac OSX Snow Leopard bug seems to be rather deadly as it leads to a loss of all user account data. Apple's support forums describe the problem as occurring if a user logs into a Guest account on their Snow Leopard computer, and then logs back into your regular account.
The solutions are few, but the good news is that Apple has declared that they are indeed aware of this issue and are working on fixing it. They also state that the bug is extremely rare, although the severity should count towards the quick release of a fix.
Currently if you have a Snow Leopard computer, it seems it is best to disable your guest account, or atleast disable and reenable it if you need to actually use it. For those with backups on their Apple Time Capsules, CNet has a way to recover your account.
If you thought that would be a good reason to be an Apple Time Capsule, think again. This touted backup device seems to have random failures where the devices just dies after over a year of operation, something which is quite obviously undesirable in a backup device.
Our condolences to those SideKick customers with Snow Leopard backed up on a Time Capsule. The universe has just sent you a message, switch to a notebook, pen and carbon-paper.