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This is a framework abstracting away from hardware devices and image processing for particular tasks (like user/hand/skeleton tracking).
The framework itself and a sensor driver 'node' from the PrimeSense sensor of the Kinect look to be open source, under LGPL.
A set of image processing tasks seems to be available as well, under the name of NITE. This part doesn't seem to be open (in terms of source), but freely available as binary modules.
The core APIs looked to be C++, per the examples shown, however the documentation makes it clear that the C++ is actually a wrapper around the true APIs, which are plain C.
Compared to OpenKinect the Sensor piece is at the same level as that, with the framework itself, and NITE, then sitting on top.