The Samba team is working on this functionality - see this link for the roadmap and status right now:
And see a description the goal of this development:
And this excerpt from the text:
What does the Samba Team plan to do to integrate Samba servers into native Windows 2000 domains?
The Samba Team hope to gain more volunteers to help with this effort. There are basically three areas of functionality in regards to this. The ball has already started rolling with some research done by IBM's Linux Technology Center.
A Kerberos 5 client implementation in Samba that understands the proprietary MS PAC
A UNIX based Kerberos 5 KDC that can issue Windows 2000 PACs
The ability to read data from an Active (i.e. LDAP) Directory Server via the authorization credentials (TGT) received from the Windows 2000 Domain KDC
Of course, Samba is currently able to continue to operate in a Windows 2000 domain assuming that the domain supports Windows NT 4.0 File and Print servers. The new ADS (Active Directory) work has gained us much in the ability to act as a server in an ADS domain, but nothing in becoming a PDC."