acosmichippo,
Could they actually say with 100% accuracy that you were tethering? Probably not.
But they would wonder what you are doing on you [i]phone to account for the activity. They can see where you are going and what you are doing and since the iphone is supposed to be a restricted device deduce that you are likely tethering and perhaps even hacked and using applications that are prohibited.
Regardless, ATT is the service provider so they can pretty much draw whatever conclusion they want and it will be up to you to prove your case.
There are a few reports of ATT taking action in extreme cases (cutoffs and billing), but they are not likely to do so.
All that said, the odds are slim that you will ever hear from them as I would think we would have seen more reports here of ATT actions. I think that has to do with the fact that the data service is Edge and not 3G (i.e., on the slow cheap service who gives a crap what you do).
I would expect that on 3G they will be monitoring more aggresively.