AT&T will soon (August 11th?) be forcing those with unlimited data plans who tether without authorization onto a tiered data plan.
An AT&T spokesperson had this to say:
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
My wife's data usage on the Iphone 5 was no different than the 4: max was 2.5GB, often under 2GB. She's on her phone a lot, Facebook, Pinterest, email, games, some Google Maps searching, very limited...