It will likely be enabled over 3G before formal 4G rollouts occur. Even though 4G isn't here yet, I'm betting AT&T and others are already trying to increase the bandwidth to each tower in preparation for LTE, and that will have the effect of allowing more 3G traffic as well, assuming limited wireless spectrum isn't also a problem in the area.

And no talk about tethering today. As pointed out elsewhere, it's been built into the OS since 3.0. It's up to AT&T now.