The real solution to this issue will come when Windows Server 2008 is released and it's called Network Access Protection (NAP) as you can use the DHCP Enforcement method of NAP to allow only authorized clients to lease addresses from your DHCP server. For more information about NAP and its DHCP Enforcement method (and other enforcement methods) see my book Introducing Windows Server 2008.
Mitch Tulloch is lead author for the Windows Vista Resource Kit from Microsoft Press, which is THE book for IT pros who want to deploy, maintain and support Windows Vista in mid- and large-sized network environments. Mitch is also the author of Introducing Windows Server 2008, the first book from Microsoft Press about the exciting new server platform. For more information on these and other books written by Mitch, see www.mtit.com.