If you're a heavy user of MyWi, the most popular jailbreak app that allows tethering, then watch out because AT&T is coming after you. According to various reports from MyWi users, AT&T has been sending SMS and emails warning them that they have to activate the official tethering option if they want to tether their iPhone.
Additionally, the email AT&T sent to some MyWi users warns them that if they don't hear from them before March 27, those users will automatically be enrolled in the 4 GB DataPro plan, which is required for activating the official tethering option...