That is quite scary. And to dig even deeper would be the ability to predict what a user is just thinking in his or her head. Or reacting or showing customized options based on a user's mood, which could be used for both good and bad purposes. Good if it's a system that's intention is to be purely helpful to a user, such as in that computer in the movie Moon. But bad if it's system meant to manipulate users into doing things they won't necessarily want, such as convincing people that they "need" to have a McDonald's Big Mac.

There are companies already doing this. With the front facing camera on the iPhone4, you could track eye movements and even have a cost-per-glance model.