Apple's iOS now supports tethering ("Personal hotspot") although it was there months ago (see "Someone doesn’t like the Ipad Tethering" - here), and Apple's best friend offers a data plan with tethering (for extra $20/month - see "AT&T - No More Unlimited Mobile Data" - here).
So why the world had to wait for so long? It seems we were waiting for AT&T to be able to detect tethering.
This is over now. osXDaily reports that "AT&T is beginning to crack down on unofficial iPhone tethering methods, including users of MyWi, the unofficial iPhone WiFi hotspot app that is popular among jailbreakers. Customers are being notified that their service plans need updating to subscribe to a tethering plan, and that they will be automatically subscribed to a DataPro 4GB package that costs $45 per month if they continue to tether. In the email, AT&T also notes that if customers discontinue the use of tethering, no changes will be made to their plan".
See "AT&T Cracking Down on Unofficial iPhone Tethering & MyWi Users" - here (includes the letter sent by AT&T to those suspected using "unofficial" tethering).
How does AT&T do it?
See some background "Why Do Mobile Carriers Need Device Awareness?" - here and note the anonymous use-case mentioned in "Sandvine on Device Awareness (Tethering Use Case)" - here ("..one use case that Sandvine has assisted with, using device awareness, is enabling service providers to offer a special tier to those customers wishing to tether their mobile devices. Tethering is the use of your cell phone or other Internet-enabled mobile device as a modem for another device, like a notebook or PDA. It enables you to go online from your laptop, for example, in situations where there’s no other means of Internet access. By being able to identify “tethering” devices, a service provider can offer a service tier or package that caters to the subscribers’ needs).