Samba as of version 3 only do the old emulation of an NT4-domain as PDC/BDC.
AD is way more complex in its use of GP and stuff. Samba 4 can do it but its
no way near to be complete and ready for production. And btw, Vista is
working as client in both modes. There is of course possible to do some of
the GP the ugly way by enforcing registry changes via login script, but it
sure is ugly have some quirks.