Canon already have a system for reading eye-direction in their cameras, so the focus point is wherever you are looking in the viewfinder.
A high-resolution EyeToy style camera might potentially be able to determine changes, thughI doubt it'd be accurate enough.
However, this is a MUST HAVE feature so that when you are playing football and want to change player, the game senses whch player you're looking at to change to, instead of always selecting the worst possible player resulting in a conceeded goal!