In MS parlance, a road map is merely a suggestion and not worth the paper that it's written on.
early this year, near the release of Vista, MS CEO Ballmer said that releases wouldn't take as long, 2-3 year cycles. yet, only a few months ago, a leading project guy as MS was quoted as saying the next version of Windows would be "at least 3 years" from coming to fruition.
Let's do the math. Steve Ballmer say's "we'll do it in less time", the guys who write the code and deal with the internal MS struggles says "3+ years" as of mid 07 leaves us at mid 2010 at the earliest. Which of course, means 2011. Ballmer's word, the captain of the MS ship, can't be trusted.
XP will need to be supported for a long time as too many corporate folks are flat out refusing to support Vista as we can NOT support Vista because of mission-critical or line-of-business applications that won't work on Vista.
It's rather sad, actually. A company with all the resources as MS has and we're talking about the next release being 4 years away and reactive features rather than pro-active and revolutionary items. Guess they're too big for their proverbial britches.
So to answer your question, no one really knows. I'd hedge my bets on later rather than sooner.