I feel like there are always more rumors about the Nextbox 720 then there are for the other two bigs' next-gen machines (with the possible exception of wiiHD). From rumors of a next-gen Halo already in development, to stories about the x720 being forward compatible, it seems as if--even if these rumors are untrue--the R&D division at M$oft is hard at work on SOMETHING. Given there proclivity to take a head start on the competition, I would not be the least surprised to see a release in 2011 or 2012.
That said, the 360 will be heavily supported until well into the next-generation. Knowing microsoft, I also believe software updates comparable to the NXE will furthermore make the 360 a much different looking machine than it is today...
http://kotaku.com/5041835/rumor-gearbox-is-developing-halo-4-for-new-xbox
http://kotaku.com/5063787/rumor-next-xbox-to-be-forward-compatible