Administrators' profiles deleted when having Guest status

Our solution for preventing users from leaving behind profiles on lab
machines has been to add the Domain Users group to the Guests group on those
machines. As long as a user was a member of the Administrators group on the
machine, that user's profile would not be deleted.

I just discovered that with Vista, even Adminstrators' profiles are deleted
in this scenario (of course I mean users with admin rights who are Domain
Users). This happens even if the admin user's profile existed before adding
Domain Users to the Guest group.

I don't know of a Group Policy setting that will delete user profiles, other
than for roaming ones. Can anyone think of a solution for this?

(Fortunately Vista has volume shadow copies of everything by default, so I
was able to restore the important files from my profile after it was deleted!)