Vista SP1 on MSDN Subscriber Downloads
If you have an MSDN Subscription you can now download Vista SP1 from the subscriber downloads centre. You can find the related announcements on the Windows Vista Blog here and here. You can also download an Overview of Windows Vista SP1, Notable Changes in Windows Vista SP1 and, of course, the release notes.
There numerous enhancements relating to performance, reliability and security as well as application and device compatibility. All the details are in the documents linked above.
Oh, and I have it installed and it's lovely. I wouldn't swap it for another SP1.