There seems to be quite a laundry list of items coming in Vista SP2. This service pack actually covers Windows 2008 as well, in case you missed that news a while back. The advent of Vista SP1 coincided with the release of Windows 2008 in order to once again unify the kernel, ala NT4/W2K.
What's in the update you ask? Check out Mary-Jo Foley's post listing all the goodies.
The stand-alone download for this thing is probably going to be ginormous since it will include the previous service pack, so I'd suggest using Windows Update when it becomes available unless you just can't wait.