AT&T already caps its data plans at 5GB a month, but they told the FCC on Friday that if they catch you using their mobile broadband network for P2P, they'll nuke your account. Specifically, senior VP of dealing with the government Robert Quinn said that "AT&T's terms of service for mobile wireless broadband customers prohibit all uses that may cause extreme network capacity issues, and explicitly identify P2P file sharing applications as such a use."
Apparently they don't use any network management for the wireless broadband network, meaning there's nothing in place that'll slow down or stop P2P (or any other bandwidth-hogging app). It looks like this P2P ban applies to the iPhone as well (waiting for confirmation on that), despite statements earlier that its unlimited data usage was truly unlimited-so don't hold your breath for a video P2P app (like Joost or something) on your iPhone. Below is AT&T's official rationale.