Interestingly, Microsoft didn't respond to the expected request for comment with its standard "we don't comment on rumors or speculation" line. Instead, the company issued a statement with the strong suggestion that change is in the air, without actually confirming anything. See here:
Xbox 360 has found new ways to extend the console lifecycle by introducing controller-free experiences with Kinect and re-inventing the console with a new dashboard and new entertainment content partnerships. We are always thinking about what is next for our platform and how to continue to defy the lifecycle convention.
It's notable that the AAA game release schedule for 2013 doesn't extend much further than the spring. The fact that Microsoft has a March release set for its next Gears of War game is also a big red flag. Fall 2013 is looking more and more like the release timetable for the next phase of Microsoft's Xbox evolution, whatever that might mean.