AT&T does not have a tethering plan for the iPhone, therefore they can not charge you the price that they would with a plan that does not exist. Do it, but don't go crazy and watch youtube videos with your phone, otherwise you'll be headed towards a 5 GB mark and you'll raise a red flag and they'll ask why you're using so much data. It's ridiculous even though you pay for unlimited data with the iPhone, but millions of people will soon have this hack done and AT&T will get HELL if they charge people for it. Once a new firmware comes out, they'll start charging.