Windows Vista RC1 was completed today. From the Windows Vista Team Blog:
It’s official — Windows Vista RC1 is done!
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You’ll notice a lot of improvements since Beta 2. We’ve made some UI adjustments, added more device drivers, and enhanced performance. We’re not done yet, however — quality will continue to improve. We’ll keep plugging away on application compatibility, as well as fit and finish, until RTM. If you are an ISV, RC1 is the build you should use for certifying your application.
Right now it is only available for customers on the TAP program, but according to a post in the forum, they plan to make it available to MSDN and Technet subscribers.