Thermal cameras with blob tracking. cv.jit.blobcentroid object (theres a library for it). The camera being above the floor pointing down would be the best for xy tracking. less range and more lag is the kinect (cheap solution to try it out though)

Thermal cameras are pretty pricey!! You can still use CV blob tracking with a normal camera, but lighting/projection in the same space might mess you up, in which case I would go for an IR camera. I have hacked one of these with a Playstation eye camera before and its pretty easy (there's a tutorial somewhere), but that was a while ago.

As Andro says, the Kinect is an option, you can either use it as an IR camera and do blob tracking, or make use of the user tracking features of the Microsoft implementation via the dp.kinect object.

I have also seen IMU sensors used in impressive projects of this sort, you might have a look at them. There is an interesting article here, though their hardware is not cheap! http://www.x-io.co.uk/oscillatory-motion-tracking-with-x-imu/