Microsoft provided me with the following announcement today, which should be of interest:

Microsoft today announced the release of Windows Vista SP1 Beta to a select group of approximately 12K testers.  This release focuses on addressing specific reliability and performance issues, supporting new types of hardware, and adding support for several emerging standards. In addition to the news around SP1, Microsoft also announced that pre-release versions of reliability and compatibility fixes are available today on the Connect site and will later be available via Windows Update. For more information on today’s announcements, please see the Windows Vista team blog, the Windows Experience Blog  and a video by Channel 9.  In addition, Jon DeVaan, Senior Vice President of the Windows Core Operating System Division, spoke about SP1 in an August 29th QA on PressPass