One of my students this week asked about using a Samba 4 Active Directory domain with GPO's to manage the Windows XP clients. Samba 4 alpha 6 impressed me! The download comes with a small howto.txt to setup your kerberos realm and to integrate the SRV records in DNS (a chrooted bind 9.3.4). Adding users is explained, joining Windows computers works flawlessly.
Last time, Samba 4 was unable to operate Group Policies, now I tested a few, and they work! The screenshot below is no proof, but I get a kick out of it ;-)
In this screenshot, you see a fully patched Windows 2003 server in a Samba 4 domain, in its own OU, together with the user 'brel', having applied the GPO setting "remove Search from Start Menu".
The future is free!