Verizon has said that they throttle their heaviest users on their EvDO network. I'll bet AT&T's throttling is going to affect a lot more people though.
They rarely do, if ever. I myself have used over 10GB with no effects whatsoever. Every carrier has throttling terms in their contract, doesn't mean they actively do it though. Even Sprint has it that they can throttle, but they don't.
AT&T will, and so does T-Mobile. -- 2010 Ford Fusion Sport