I said it was a prediction. But it is based on following past trends. Microsoft released Halo 2 in November 2004, then after a successful run, they released the 360 in November 2009, and they used their most popular title as a flagship title to do so. They will want to do the same thing with the next xbox with Gears 3 being its standout title. So, it really comes down until how long they want to wait until they debut Gears 3. They waited 3 years between Halo 2 and Halo 3, so they could wait as long as 2011 to release a new console along with Gears 3. But that might be too long for Epic to wait. They might want to get the next game out sooner than 3 years from now, so since there has been only two years between Gears I and 2, then 2010 could also be the year of the release of the next XBOX.
Now, if MS wants to get a little head start on the competition which was a boon to them this gen and if they want to try to develop another franchise in addition to Halo and Gears, then might push harder to have their next console ready in November of 2009, and this would follow the precedent set by the four year interval between the 360 and the original XBOX.
Either way, I think there will be concrete information released by MS at the next GDC (which seems to be the conference where Microsoft likes to make their major announcements), Gears of War 3 will be the game they use to sell their next console, and the two will release fairly close together.
There have been many little pieces of information released this year that makes it look as if Microsoft has been considering different facets of the development of their new console. Such as leaked rumours about what kind of storage medium it was going to support, etc.
I found several links that said Microsoft was working on plans for the next xbox as early as January 2008, so even though I would think a 2009 release would be too early, if they were working on it even conceptually that long ago then it could be next year.
http://www.1up.com/do/newsStory?cId=3165235
http://www.stuffwelike.com/stuffwelike/2008/08/25/gearbox-working-on-halo-4-for-next-xbox/
http://www.tgdaily.com/content/view/9704/139/
My most anticipated games: Whatever Hideo Kojima is going to do next, Final Fantasy XIII, Final Fantasy Versus XIII, Gran Turismo 5, Alan Wake, Wii Sports Resort. Cave Story Wiiware.
