Looking to remind people, hey, we made this thing, Microsoft's upcoming Kinect software development kit for the PC (which anyone can grab) will include a variety of features most hackers and code enthusiasts haven't been able to manage on their own.
One of those features is the ability for the camera to track the skeletal movements of two people, not just one. It'll also give people access to all four elements of Kinect's microphone, along with its noise cancellation features.
Other things you'll get with the software development kit (SDK) include a ton of documentation, speech recognition tools and "depth data, which provides the distance of an object from the Kinect camera".
So...you could jerk around with code the hard way, or grab this and let Microsoft do some of the work for you.
Before you ask if this will help, say, games, unless they're free the answer is "no", as the SDK can only be used for non-commercial purposes.