Using a variation of the technology used in optical trackballs and mice its possible to track eye movement and position and focus.
With CAUTION (you do not have to try this) try holding your mouse to the outside corner of your eye, with the light shining on the white of the very outside, taking great care not to expose your pupil to the light emitted, you can get quite close, but you DO NOT need to touch your eye. Holding the mouse in place stare at your screen and move your eyes around. it does actually work but obviously not very accurately, and you can't use a light that intense close to you eye for any length of time without causing harm. A few changes to the electronics, mainly the light source by changing to either a very low intensity and focused visible light so as not to distract, or an invisible light source like infra red. The CMOS sensor should obviously be viable with the chosen light source and should have a higher resolution than a standard computer mouse, along with a higher operating frequency as in high-end gaming mice. A new set of tracking algorithms and it should be up and running ,after a lot of work.
Calibration would probably be best performed on the fly, as the surface of the eye is far from smooth and around the edges of the iris deforms quite a bit into the lens.
There could be additional benefits as well as eye tracking, including biometrics and physiological feedback of heart rate, oxygen levels and who knows what else.