Nope. Knowingly and willfully violating the terms of a contract is an unfortunate by-product of that contract being adapted after the fact by ATT without giving me reasonable options to use what I pay for.
How was the contract "adapted after the fact"? Are you saying that when you signed up for AT&T your data plan contract allowed tethering, and then they changed the contract to forbid tethering? As far as I know AT&T's basic data plan for the iPhone has always had a clause forbidding tethering. Whether it's reasonable or not for AT&T to charge extra for tethering is a separate question from whether they've reneged on a contract as you seem to be implying.