SP2 has been released for Windows Vista and Windows Server 2008. Both OSs use SP2, even though this is the first service pack for Server 2008—Server 2008 came with SP1.
SP2 adds support for a new Bluetooth specification and burning Blu-Ray discs, but generally doesn't change the experience of using either OS. There is one interesting addition with the service pack, though. SP2 adds Hyper-V to Server 2008 systems, so a machine with the service pack won't have to download Hyper-V separately.
If you don't want your users to install the service pack, make sure to use the Windows Service Pack Blocker Tool Kit. The same tool kit can be used for the new service pack as for previous packs.
The standalone SP2 is available here from Microsoft. The service pack is also available through Windows Update. Microsoft has provided a list of frequently asked questions about SP2.
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