Sounds good in a Utopian world but this is AT&T we're talking about. They'd throttle everyone just to save bandwidth. Their "algorithm" would be a trade secret so no one could technically pin it on them.
Actually, I was thinking T-Mobile, not AT&T. They seem more likely to try a radical change to the existing pricing models. And to some extent, they do something vaguely like this, where after you exceed your bandwidth cap, you aren't cut off or charged more, but reduced to EDGE speeds (I'm hoping at the very least after they're done with the LTE rollout that the reduction would be to 3G speeds, but we'll see what happens), so it's not entirely unreasonable for them to consider the idea of this kind of concept.