Microsoft has implemented their DNS registration mechanism to be flexible w/ respect to the DHCP client's ability to perform its own dynamic regsitrations. The default behavior of the Windows 2003 DHCP server is to perform registrations of only the client's PTR record. The client itself performs the registration of the "A" record (by default in all Windows versions that support dynamic update-- 2000 and newer).
Old DNS entries can be removed via aging and scavenging, typically. The DHCP server can also remove the client's "A" record by using the "Discard forward (name-to-address) lookups when leases expires" option.
You can use the "Dynamically update DNS A and PTR records for DHCP clients that do not request updates (for example, clients running Windows NT 4.0)" option to cause the DHCP server to register both "A" and "PTR" records for clients that do not support dynamic update.
A slightly more in-depth article on these topics is available here: http://support.microsoft.com/kb/816592