In theory, the GPU in the Pi can easily handle the floating-point calculations needed to do skeleton tracking in near-real-time using the Kinect's data - that's part of what the GPU in the Xbox 360 is doing. I haven't looked at the OpenKinect software, but, the majority of the work should involve matching up calls from OpenKinect to floating-point processes in the GPU. If no one else has cracked this nut within the next year, I could do it, but, I have other pokers in the fire at the moment.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!