From a blog entry that quotes an article by Paul Thurrot:

Windows Vista Beta 2: According to Paul, Beta 2 has to be "feature complete" by the end of September. This probably means whatever features they want to include in Beta 2 needs to be working in whatever build they come up with by the end of September. From there, they do internal testing I believe and internal bug fixes before releasing to the general testing public (us). Supposedly, we're suppose to expect Beta 2 on December 7th, 2005.

Windows Vista Beta 3: At this time, Microsoft does not have a third beta planned for testers. But this could change - this is just my thoughts here.

Windows Vista RC0: Expect to see this puppy to come running in on or around April 19th, 2006.

Windows Vista RC1: June 28th, 2006 is the date for this one.

Windows Vista RTM: Vista will be released to manufacturing on August 9th, 2006 with a broad release expected in November 2006.

The "Longhorn Server" Development will follow the same release schedule for the betas but they do their own thing when it hits RC0.