There have been a few rumours over the last few months about the NextGen consoles. A 2014 release seemed to be the consensus, but also talk that the next XBox would be announced next year (2012) possibly at E3.
Two more recent rumours:
Nexrt XBox is codenamed "Loop", it will have a main processor and additional assisted cores for graphics, AI, physics, sound, networking, encryption and sensors. It will be smaller and have a cheaper price to encourage adoption of Kinect. (Original MSNerd link). Interestingly, the GamePro article that links to MSNerd states that they think this means the next XBox will not be released until 2014. This ties in with Microsoft stating the 360 would have a ten year cycle (this would have it end 2015).
Contrast the above with this rumour which says the next XBox will be integrated very closely with Windows 8 and will be released for Christmas 2012. Also is a quote from the CEO of the team that developed the Anniversary of Halo:CE where he states he would be "shocked" if Microsoft doesn't launch its next gen before Sony.
Not sure how to read what seems to be quite a difference in dates.
Update 21 Nov: Analyst says it would be "silly" to launch next XBox before 2014.