I have a little story for you why AT&T does not care and will never catch wind. Back in January I tried my hardest to get MMS enabled on my phone and I had to talk to AT&T reps to get them to enables WAP Push on my account (which is blocked for iPhone users so they can't get MMS) A few reps said they simply couldn't do it because their computer wouldnt let them, however I knew this was a lie because other people had done it and it worked. Finally I got a woman that did not know what I was trying to do and she finally enabled it and MMS worked fine. Over the next few months I had tons of MMS messages that were on my bill. It literally said MMS text message and it even had it's own little section. Also I had tethering on my phone and had ridiculous 2-3 GB data usage numbers. If you think AT&T is EVER going to get wind on your account that you're doing something, they simply won't because they don't care. For the case of MMS, it says right on my Bill, iPhone MMS Messages Sent/Received. How simple is that when you "couldn't" even send MMS on the iphone at that time? With the case of tethering, they can ONLY go by the amount of data you use. It's not labeled on your bill it just adds to the total. And how can they tell you werent just using your iPhone ALOT on the internet? They can't. So if they won't stop something like MMS that's written RIGHT ON THE BILL, how are they going to get "wind" of you tethering? I think it's silly all these people scared of getting caught by AT&T when they can't even get wind of tethering in the first place unless you go over the softcap of 5GB.
TLDR: Don't worry AT&T won't get wind of anything because they don't care.