The problem, can easily be reproduced when a user logs into the 'guest' account, either on purpose or by accident, and when they log back out of the account and back into their normal one, they find that their account has been fully reset with all data wiped and lost - the account is like a brand new one. The home directory still exists under "/Users/username" but is completely empty.

It's not clear how many users are affected, but it seems like any user who had Leopard before the upgrade, and had the guest account enabled are affected and are at risk.

Thankfully, I am using Time Machine which has come to my aid a few times since Leopard came out.

Yup, Time Machine is a Steve-send.