Beginning August 11th, AT&T plans to force users to join a new data plan if they deem the customer to be "illegally tethering". It seems that this is mostly targeted toward jailbroken iPhones with wireless hotspot apps. Upon further reading, it seems like AT&T is less concerned about the tethering itself, and more so about making your unlimited data available to several people at once.
Here is one of many similar articles about the topic.
