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.
Mobile wireless broadband service relies on shared network resources. With any shared network, some limitations on uses of individual subscribers are inherently necessary to ensure that all customers collectively receive an acceptable level of service.
Unlike wired broadband networks where the maximum number of potential simultaneous users in a given neighbourhood is known in advance, the maximum number of potential mobile wireless broadband users that may simultaneously seek to access a given cell site at any particular time - and thus the collective service experience for all users at that site, for both data and voice services - is far less predictable due to the inherently nomadic nature of mobile wireless users.
In order to provide quality service to all our customers, our terms of service for mobile wireless broadband do prohibit uses that may cause extreme network capacity issues, including P2P file sharing.
Our terms of service are similar to those of other wireless providers.
Use of a P2P file sharing application would be a breach of contract that would allow us to terminate a user's service. That said, since the vast majority of customers abide by their contracts, we have so far found it unnecessary to terminate anyone's service for this reason.