I'm guessing it's like the wiimote, which has an IR camera, but you have no way of getting the raw IR image. There's an onboard processor that analyses the image from the IR camera, does blob tracking and stuff like that, and then gives out an image with
up to four dots. I'm guessing Natal has a similar processor that gives you the individual joints it sees and perhaps an ID for the face it recognizes (as well as a simple RGB image, see the video chat or the reflection in the water in the Milo demo), but not
the actual raw grayscale depth-image.
I'm guessing it's like the wiimote, which has an IR camera, but you have no way of getting the raw IR image
I think you do have access to the image buffer... You saw the facial recognition demo... I guess the camera can always hash your face and send it to the Xbox... but I think MS has bigger plans for the camera than just as a motion controller.