In what increasingly hints at Microsoft moving to a subscription based distribution for all their software, MS CEO Steve Ballmer said that the 5-year gaps between each release of Windows is too long and won't happen again. While it's not always 5 years, it's still fairly long. This points towards subscriptions, because with major changes in Windows requiring upgrading comes the cost involved in doing such. You can much more easily convince a business to buy 150 licenses of “Windows 2010” if they get it for a subscription fee of some sort. This could mean that Vista's successor could come much sooner than Vista itself arrived:

Microsoft has not said when Vista's successor, code-named Vienna, might ship or what features it will contain. Despite the desire to make Windows releases more frequent, Ballmer said there will always be people trying to work on things that take years to make happen. "We just won't promise them to customers and hold up big releases," Ballmer said.

As an interesting tidbit, Ballmer said in the same speech that a miniature Sony, Yahoo and Google lived inside Microsoft, aspiring to come out. Sounds creepy.