Well, technology required is not anything new, here is example:
http://www.youtube.com/watch?v=Jd3-eiid-Uw

However using camera to detect user-screen relative position is a bit harder, tho not impossible. And it is surely not a hardware dependent.

On a sidenote tracking eye movement is useless as the angle your eye moves is too low to make a difference, you just need to detect a relative head position to a screen.

Also I don't see much use for that technology in a tablet.