According to some recent estimates, by the end of this year Vista will have been deployed in 9% of businesses and still won't reach 1/3 of the market by 2010. On top of that, factoring in downgrade licenses it seems that Windows XP is still outselling Vista. That must hurt Microsoft, especially when very large vendors have mentioned they simply will not deploy Vista, waiting instead for Windows 7.
Interestingly, Windows 7 has been in the works for a while and it's been rumored more than once that it will be released next year. That's just speculation, of course, and given the dollar investment Microsoft has poured into Vista I doubt they will just sit on their hands.