If ATT is gonna change my plan on the "assumption" that I am tethering, I am gonna ask for proof. They will need to show me some kind of logging to prove that my data traffic came from a source other than my iphone.
I think we should all ask for such proof.
And who is to say your phone did not get some kind of hack or virus that enabled someone to make it appear that traffic was coming from your iphone's ID?
This seems like a very difficult assumption for ATT to really "prove".
They don't have to prove anything. They can change your plan at any time for any reason and there's not a thing you can do about it, other than complain and rant and rave about how you will sue and yada, yada, yada.