Samba-3.0.0 is due out around April this year. It has the ability to use Kerberos and can join a native Active Directory security context. Samba-3.0.0 also provides new migration tools from NT4 style domains to a Samba environment. It has a new multiple back-end ability that means you can mix a Samba tdb based SAM (Security Account Manager) database, with an LDAP back-end, etc.
Samba-2.2.x can act as a "primary' domain controller, as well as as a "fallback" domain controller. I do not like to call it a "back-up" domain controller (BDC) as it functions quite differently from an NT4 BDC.
Samba cannot at this time act as a BDC to an NT4 PDC, or vica versa. The domain control technology has been very stable for at least two years now.
This was first published in March 2003