I was really surprised when I saw this video of Logitech's Orbit MP webcam. It's about $100, and it has impressive face tracking algorithm. I knew that people were making headway in face tracking algorithms, but I didn't think it was this good.
There are now open source tool kits for augmented reality. However, they use a template black and white pattern as a reference platform. It is definitely possible to use people's faces as a tracking platform. Then, you can put statuses above people's heads, like their names. Or in a augmented reality game, their "hit points".