It's pretty close to happening; they can already track your eyes fast enough to render at higher resolution at the fovea area as your eyes move around:
http://research.microsoft.com/en-us/um/redmond/projects/fove...
Also, your eyes are very slow to focus relative to your eye movements, so it is probably doable now. It still will not feel like real life, just sort of eye tracked depth-of-field. Your eyes will still be focused at infinity, but near things will blur when you look in the distance, and vice versa.
