I just reviewed the technical overview for the upcomming service pack for windows server 2003.
There will be some new features introduced and lots of enhancements.

One the caught my eye is : Service Pack 2 provides performance improvement for SQL Server 2005 under intensive workloads. Service Pack 2 installation installs these improvements by default.

Others I like are Enabling ‘Firewall Per Port’ Authentication, Improved IPsec filter management, Performance Improvements under Windows Virtualization (finaly blazing fast vpc's???)

Windows Server 2003 Service Pack 2 is especially important for organizations planning on testing and deploying Windows Vista and Windows Server “Longhorn.” SP2 contains important enhancements to support the deployment and management of Windows Vista and Windows Server 2003 SP2.

one thing to keep in mind: it needs lots of diskspace:

Check the disc space requirements. The following are the approximate storage requirements for the installation of SP2.

Find more info here: http://www.microsoft.com/technet/windowsserver/sp2/overview.mspx

//Bart