Microsoft position on this early release, stick to their official company statement that Windows 7 will be available within three years of when Vista shipped, which would put an early 2010 shipping date of the new OS. This intends for deliberately to avoid in times of unexpected event of delayed release.
With Microsoft struggling on Windows revenue and the exploding lines of netbooks in the market that preferred Windows XP(rather than Vista with SP2 coming this Q2) affect negatively Microsoft software business. The following reasons why Microsoft is hesitant for Windows 7 release this year.
Now we see how Windows 7 have improved both on user interface functionality and performance and gone stable on proven benchmark. I believe it will be polished more to run smoothly on netbook-level hardware, as the current trends suggest a market on mini-PCs.