Modern eyetracking solutions can be quite accurate. I've used SR Research's Eyelink II [1] and have seen results from Tobii [2] eye trackers that are impressive too, considering their passive nature.
The SR Research I can totally understand, since it's head-mounted, proprietary hardware. The Tobii is a bit more impressive, but I think it's using IR emitters to light up the pupil, and I assume it's using their own camera. Very impressive monitoring rates, though.
My doubt concerns claims to track eye movements reliably using a built-in webcam, and available light.
My doubt concerns claims to track eye movements reliably using a built-in webcam, and available light.