AT&T is ruining a lot of people's days with a customer mailshot explaining that its "records show that you use [tethering] but are not subscribed to our tethering plan." iOS, of course, will disable the built-in tethering facility if you do not have an appropriate carrier plan.
There are a few jailbreak apps, the most popular of which is MyWi (previous TUAW coverage), that bypass the plan check and enable tethering independently. When you run MyWi or similar apps, your iPhone creates a wireless hotspot that allows you to connect other devices without the explicit permission of your carrier. Until now, people have assumed that AT&T either doesn't care or cannot determine that the traffic comes from a connected device rather than the iPhone itself. Clearly, those assump
