A few people on the Apple discussion boards have found a problem with Snow Leopard, where the system has deleted their home directories after logging in to the guest account.
Apple discussion poster "Bas B" writes:
"I upgraded from Leopard to Snow Leopard last week. By accident I clicked on the 'guest log-in' icon on my MBP 30 minutes ago. After about 1 minute it automatically returned to the log-in screen, without showing up the guest desktop, so I decided to click on the 'guest log-in' again. This went pretty smooth and when I logged out the message appeared that all the 'guest log-in data' wouldn't be saved, but returned to normal. When logging in to my regular account, everything was gone. I don't use filevault. Nothing has been renamed to something else. My home-directory still exists under /Users/username but it is just empty."
Other people have reported a similar situation, where they log into the guest account and then log into their regular accounts, only to find all data gone and the accounts reset to the default "new account" look. This could be due to a bug in how guest accounts are managed since data and settings are deleted from these accounts upon logout, but also could be from some corruption in the guest account. So far, it does not seem to be a widespread problem.
If this happens to you, immediately restore using your latest Time Machine backup (or other full system backup), and then go to the Accounts system preferences and disable log-in on the guest account.
This problem does not happen on my Snow Leopard machine (upgraded from Leopard) upon enabling guest log-in, which suggests one possibility could be some Snow Leopard incompatibility with how the guest log-in was initially enabled. If you had it enabled in Leopard before upgrading and the problem happened without any changes to the guest account settings, then it could be from an incompatible setting between how Leopard and Snow Leopard handle the guest accounts. As such, try going to the Accounts system preferences and toggling the guest log-in setting to see if that refreshes the way the system is handling the guest account.
If you need guest account functionality and do not trust the built-in account because of this problem, for now just create a new non-administrator account (call it "Visitor" if you need a semi-decent alternative name) for use as a guest, and customize restrictions for it with parental controls. In most instances this will work just fine, since the only real difference in behavior for guest accounts is that data and settings are reset upon logout.