Via Windows Vista Team Blog:
SP1 will contain changes focused on addressing specific reliability and performance issues we've identified via customer feedback, supporting new types of hardware, and adding support for several emerging standards. SP1 also makes additional improvements to the IT administration experience. We didn't design SP1 as a vehicle for releasing new features; however, some existing components do gain enhanced functionality in SP1.
More information on what's included in SP1 can be found in the detailed white paper.
A Beta release of Windows Vista SP1 is slated for availability in the next few weeks (only for a small group of tester); a later pre-release of SP1 will be available to a larger group of testers via MSDN and TechNet subscribers.
We're targeting releasing SP1 to manufacturing in the first quarter of 2008, but as always, we're first and foremost focused on delivering a high-quality release, so we'll determine the exact release date of SP1 after we have reached that quality bar.