Based on the amount of Apple forum postings on this issue, it seems to be very serious: there is the possibility of data loss if you have a guest account enabled prior to installing/upgrading Snow Leopard. It is speculated that the actual bug is that after logging in to the Leopard-created guest account while running Snow Leopard, your regular account gets treated like a guest account – as in, all data deleted after log out, settings returned to default, and so on – nasty.