Update (10/13/2009): In a short statement yesterday, Apple finally acknowledged the data-loss issue plaguing Snow Leopard, claiming the problem occurs only in "extremely rare cases" and that they are already working on a fix.
It appears that Microsoft isn't the only one facing serious backlash from its users over the loss of data. According to ITWire, complaints on the Apple Support forums have been cropping up the last few days due to a major bug in OS X Snow Leopard that can reportedly lead to all user settings being reset and most data wiped out from the hard drive.
The flaw is triggered when a user signs into the guest account, be it intentionally or unintentionally, and then attempts to sign back into their regular account only to find that all of their files have been erased. Unfortunately, the only way to get them back is from a backup on a separate drive.
The number of users impacted by this bug is uncertain as is the exact cause behind it. Speculation is that something makes Snow Leopard treat the regular account like a Guest account, from which by default all data is deleted upon logout. Furthermore, it seems the problem occurs only when the Guest account was already enabled in Leopard before being upgraded to Snow Leopard.
Apple has yet to address the issue, but until the company delivers an update it's probably a good idea to disable the Guest account altogether.