Ask HN: What would it take to detect someone's focus on a computer screen?

The Jeff Atwood post on the Microsoft Surface is on the front page right now. There's discussion in the comments about touch + keyboard being 'the future'.

I wonder if there are alternative ways to interface. In particular, I'm almost always looking where I want to focus. Why should I have to move my hands to tell the computer what I want? My brain automatically reveals what's important to me by directing my pupils at it. How many cameras would it take for a computer to reliably pick this up? What would be the constraints? Lighting? Distance? Screen size?