Don't do it. You will definitley be caught by AT&T if you do that. Just use your normal internet it will be faster. This will be abusing the tethering use cause you will be exceeding your download speed greatly and your download. You don't want to**** with AT&T when this happens, they have the right to give you fat-ass bill if you do this.
Don't get me wrong, i use torrents 24/7, downloading a couple tv shows right now, but ****around with it while your iPhones tethererd.