Customers lucky to still have unlimited data plans are putting it at risk if tethering. AT&T has confirmed that it will revoke unlimited data plans held by users who are using a Jailbroken iPhone for tethering. It will then lump its 4GB tethering-approved plan on the customer. The carrier has sent out warning letters to those tethering without a plan.
@Yungjohnnybravo @TatWZA

In order to keep tethering, a customer will have to switch plan, but since the unlimited data plan no longer exists, that means taking on a 4GB limit split between smartphone use and tethering. The minimum tethering plan would actually cost a customer with unlimited data an additional $15 per month.

Customers lucky to still have unlimited plans could be forgiven for getting angry about this policy, as there was no state restriction on tethering at the time the unlimited plan was still available.