The big blue hammer is starting to fall on iPhone users who jailbreak their devices and use them to tether other devices to "unauthorized" data plans (read: unlimited data plans).
AT&T customers who use their device for tethering but are not on a tiered data plan -- something you can only accomplish if you have an unlimited data plan (grandfathered in from back when there used to be such a thing) and you jailbreak your phone -- are about to get tiered, like it or not.