Let's clarify. They may determine that you are using an iPhone and adjust your data plan. That is their right according to your contract, and you have no recourse if they do. From what I read on these forums, they don't seem to do that often -- though that may change now that they have changed their data plans.
On the other hand, the terms of the contract you signed with AT&T specified that you would not use an iPhone on your data plan so you would be violating the contract. If, you know, your word matters to you.