Now it seems to be too late for those caught in the MyWi act, as an AT&T spokesperson commented:
"Earlier this year, we began sending letters, emails, and text messages to a small number of smartphone customers who use their devices for tethering but aren’t on our required tethering plan. Our goal here is fairness for all of our customers. (This impacts a only small percentage of our smartphone customer base.)
The letters outline three choices:
1) Stop tethering and keep their current plan (including grandfathered unlimited plan)
2) Proactively call AT&T or visit our stores and move to the required tethering plan
3) Do nothing and we’ll go ahead and add the tethering plan on their behalf — after the dated noted in their customer notification"
The problem is that many users on grandathered unlimited data plans are using jailbroken iPhones and tethering data across the board as if this was their cable Internet connection. If you read our article about smartphones and data, you'd see that carriers are reaching a tipping point with their capacity to serve so many ones and zeros, so they are tiering data, and exploring every option to limit consumption, including caps for the heaviest users on AT&T after October 1st.
All in all, if you keep on tethering and are not on a suitable plan, you will be moved to one automatically after the date AT&T will send you. The rumors have been for August 11th as cutoff date, but that hasn't been confirmed by the carrier.