With Kinect still in its first year, it is understandable that Microsoft would maintain a position that Xbox 360 still got a long life ahead of him and that they have no plans to release its successor any time "soon". On the other hand, the company has to actively work on researching and developing a viable successor to be ready to release it when the time comes.
The first solid proof about that R&D process is not actually a single piece of evidence, but rather four.
Recently, four Microsoft employees have updated their LinkedIn C.V.s separately to reflect their experience in developing the Xbox 360 successor.
The first employee is Jeff Faulkner who held the title of "creative director of Xbox Next-Gen" from January 2011.
The second employee is Jonathan Harris who has been the "senior creative director" of Xbox Next-Gen since September 2011. In his C.V., Harris revealed that he is working on "Designing the next generation of entertainment."
Next in the list is Patrick Corrigan who takes part in providing UI and branding support for "next-gen XBOX on all forms of media."
Finally, an Xbox hardware intern called Joe Langevin is helping to prevent "electromagnetic interference in next-gen devices."