An app called MyWi allows you to tether your iPhone or iPhone 2 without AT&T’s explicit permission, a situation that has led the carrier to send a stern letter to tetherers requiring that they stop tethering or automatically be added to a $20/month DataPro plan.

Why such draconian measures just for using data that they already, technically paid for? Well, according to AT&T’s contract using MyWi and other apps is, in short, forbidden in the carrier contract.

TUAW has an excellent piece on the whys and wherefores of this move and notes that the problems associated with offering massive streams of data to devices like laptops and Wi-Fi tablets will reduce throughput for everyone, creating a virtual tragedy of the commons. Regardless of which side of the argument you fall, AT&T has finally decided to take a stand and they’re playing for keeps.

via TUAW