Selective dimming lights

On the way home tonight, I was wondering - you dip your lights as other cars go past.
But in the grand scheme of things, a 'headlight' isn't overly different to a backlit LCD - you _could_ use an LCD to make 'light' and 'dark' patches in your headlamps.

So I think it should be possible to have a (directional) light sensor, that 'dips' the section of your headlight that's pointed towards another light source (like - another car).