http://www.techist.com/forums/f9/microsoft-windows-release-cycles-203118/
Right in the Read First Sticky.
Microsoft has always done a 2-3 year cycle. They would have had a OS out before Vista but there was a failure of the code. Codename Longhorn as it was known before it became Vista was in progress at the time after XP and it also had some great features. But sadly Microsoft bit off more than they could chew and the code line failed.
At was at that point that they started work on what is now Vista and less than 2 years after they started work on it, it was released to the public. It was premature at best of a decision as it hurt them greatly. But they are trying to fix that with Win7. At this time it is already confirmed that they have started work on several of the new features to be included in Win8. Which at this point is about 2-3 years out as it is.
You wont see a Beta of that hit for at least another year.