From Paul Thurrott:
"A source close to Microsoft today sent me documentation about an Update Rollup for Windows 2000 that the software giant will release in 2005 in lieu of a Service Pack 5 (SP5). "The Update Rollup will contain all security-related updates produced for Windows 2000 between SP4 and the time when Microsoft finalizes the contents of the Update Rollup, and a small number of important non-security updates," the documentation reads. "Because Microsoft believes the Update Rollup will better meet the needs of customers than a new service pack, there will not be a Service Pack 5 (SP5) for Windows 2000. Therefore, SP4 becomes the final service pack for Windows 2000." So there you go."
News source: Windows IT Pro