Service Pack 2 for Server 2008 and Vista, the first service pack Microsoft will be releasing simultaneously for both the client and server versions of Windows 32-bit and 64-bit, is officially slated for the first half of 2009, long before Windows 7. Microsoft says that a more concrete date will be chosen based on the feedback the company receives during the beta testing process. Soon after the first beta build was sent out, Redmond told testers that it was planning a Release Candidate build and a final build afterward, but did not give any details beyond that. Tech ARP claims the company has already narrowed down its timeline (still vague dates, but they're much better than "the first half of 2009"):
A confidential source has revealed that Microsoft will deliver a Release Candidate in February, 2009 with the final SP2 binary scheduled for RTM (Release to Manufacturing) in April, 2009. As always, do remember that when Microsoft announces that a product is RTM, it doesn't mean you will be able to download it right away. RTM merely means it is "released to manufacturing", including OEMs. Actual public release will be sometime later. Release to web (RTW) will also occur sometime after RTM.Microsoft has also told testers that it currently has no plans to give them SP2-slipstreamed versions of Vista and Server 2008 at the end of the beta program. SP2 slipstream media will be released to MSDN/TechNet subscribers, Volume Licensing, and OEM channels though.
I would also like to address some (incorrect) speculation recently on why SP2 requires SP1. Some have said that this is because SP2 is currently in beta, but this is not the case. The final version of SP2 will still have SP1 as a prerequisite. The reason for this is size. Microsoft wants the size of SP2 to be smaller (if SP2 was cumulative, it would be huge, considering how large SP1 was). As well, since Server 2008 shipped with SP1 already installed, including the contents of the SP1 client code would add make the Server deployments even bigger. By releasing one single service pack, Microsoft has less testing to do since Vista and Server 2008 have the same binaries for all common files, meaning a quicker release.
Source: full article@arstechnica