Microsoft has posted download links to Windows Vista SP1 RC Preview on the MSDN homepage for MSDN subcribers. "Get the latest preview of Windows Vista SP1 on MSDN Subscriber Downloads. This new release of SP1 addresses reliability and performance issues, and provides support for new hardware and several emerging standards." Windows Vista SP1 is slated for an early 2008 release but beta testers can get their hands on early preview builds. Users have reported that the service pack not only includes many bug fixes but several performance enhancements as well.
Update: For reasons yet unknown, Microsoft has pulled the announcement at MSDN. Microsoft were unavailable for comment.