[mikkojay]

This approach would require a PC for processing behind the scenes, but if you are into .Net:
http://www.aforgenet.com/
It is an open-source image processing library. I have used it to detect objects with a webcam in real time.
Many of the examples out there deal with color filtering, but if you could control the ambient lighting in the webcam's view, you could use "blob detection" only.

-Mike

 

[mikkojay]

How far away do you imagine people would be from this in the typical scene?
These IR proximity sensors can see out to about 5 feet. You could position 2 of them at 45 degree angles fanning out in front of the picture. You could then poll the 2 analog voltages from the 2 sensors, compare them, then calculate the best position for the eyes to look.
That might be a simple approach without needing to get too fancy.
-Mike