Check that the two users that get their tables created in dbo are not somehow in the sysadmin Fixed Server Role, either explicitly by a named login (but you said you didn't create them that way) or via another group, e.g. BUILTIN\Administrators (hopefully you do not have this group since its a new 2008 install) or some other group.
There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato