Hmmm, doesn't happen on mine. Have to look into this.
EDIT: From what it sounds like, if u have allowed Guest login access for Leopard, and upgrade to Snow Leopard, on use of the login "Guest", SL turns ur regular account into a Guest also. On logging out, Guest accounts get emptied.
Nasty tbh. Mines not an upgrade, so that could be the reason it don't happen.