It seems the only work around at this stage is to disable the Guest account, or at least disable it and then re-enabling it so that it's a native Snow Leopard account. Another suggestion is to create a new account and enforce parental controls, if you really need a temporary account.

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.