Samba 3.0's net command internally knows how to lookup a PDC, but will not fall back to looking up BDCs for certain operations. For example, user listing, but not changes, will not require a PDC. More importantly however, a 'net rpc join' may succeed against any Win2k server, even if it is not a PDC emulator. We should join an available BDC, rather than failing.
'net rpc join' doesn not have the knowledge whether it is talking to a AD domain or an NT4 domain. That is implicit in the 'rpc' command portion. And it also does not know if all the BDC's in a mixed mode domain are WIN2k or later. If you want to open a bug for user listings being able to talk to BDC's that would be fine, but the join procedure will stay as it is for now.