From the OCS R2 deployment documenation:
1) "All domains in which you deploy Office Communications Server must be raised to a domain functional level of at least Windows Server 2003. You cannot deploy Office Communications Server 2007 R2 in a Windows 2000 mixed or Windows 2000 native domain."
2) "All forests in which you deploy Office Communications Server must be raised to a forest functional level of at least Windows Server 2003".
I believe if you raise the forest functional level to Windows 2003, you cannot have a Windows 2000 native child domain though...
Curtis