This is a really good question.
It is true that Windows 2000 doesn't have the same pdc/bdc structure you are used to with nt 4. It runs with a domain controller rather than a PDC... the domain controller being the 'godfather' of the domain. It grants priveleges, and has to have AD enabled... As for your 'bdc' unlike NT where a domain can only have a single 'master' model, active directory in 2k allows for a multimaster replication model... but you need active directory on a domain controller to do this... so you could only have a member server and not a true 'bdc' that you are looking for.