UPDATE: Apple is aware of this problem and is working on a fix!
A number of reports on the internet are now covering the problem where user accounts are getting deleted after people have logged into guest accounts on their system. We covered this a few weeks ago in an initial report, followed by a secondary report that discusses some additional details of the issue and potential fixes.
Here are some summary points from the previous articles:
For affected systems, the problem seems to happen only when guest accounts were enabled for login under Leopard before updating to Snow Leopard
Guest accounts enabled in Snow Leopard work fine.
Disable guest account login if you do not use it
Use a standard "managed" account as a guest account alternative
Disable guest account and then re-enable to clear problems with how the system is handling it.
Are you at risk?
As long as your system does not have the guest account enabled then you should not be concerned. However, for people who depend on the guest account, be sure to fully back up your system before trying it out. Use Time Machine or a drive cloning utility to make a complete backup, and then test out the guest account on your system. The guest account works as it should on many computers that had it enabled in 10.5 Leopard before upgrading (and so far does not seem to show problems when it was enabled in 10.6 Snow Leopard), so this is not a universal problem with Snow Leopard installations; however, it's best to be safe and have a full backup before trying the guest account on Snow Leopard.
CNET Editor Erica Ogg has a recent article on this problem, stating Apple has acknowledged this problem and claims "We are aware of the issue which occurs only in extremely rare cases and we are working on a fix." We agree this is not a widespread problem, but it is a serious one that people should be aware of, and we do caution everyone to take the extra precaution and have a full backup of their systems.