After samba 4 long run, now it near to alfa stage and it already support group policy. There is no exactly date when will issue the alfa (or beta) version, but I have a technology preview 6 live cd which content 4 group policy (under 4 ou) + 1 domain policy and up to 6 user all applied difference policy.
Have a try and see how well samba 4 can do (At this momment, printing and security is not consider at all)
Dowload samba 4 live cd
(This is my 2nd samba4 live cd)