Service Pack two is a Release Candidate (RC) for Windows Server 2008 and Vista. There will be two images, one for x86 and one for x64. Either installation will work for both Windows Server 2008 and Windows Vista because they have the same underlying system; that’s why W2008 is Windows Server 2008 Service Pack 1 already is you check it in WINVER. It’s available on MSDN and TechNet now to will be available to the general public next week.
This is not a RTM release (just a test) so don’t install it on production systems.
You can learn more about the changes here. The big one is that the RTM release of Hyper-V is included. I guess we’ll get Windows Vista with SP2 and Windows Server with SP2 media not long after RTM to simplify new builds if you are in a license scheme. There’s also some better power management features.