Nicholas Weaver has been speculating on how AT&T are planning to implement their promise to filter illegally torrented copyrighted materials on their networks.
"All that is necessary is that the MPAA or their contractor automatically spiders for torrents. When it finds torrents, it connects to each torrent with manipulated clients. The client would first transfer enough content to verify copyright, and then attempt to map the participants in the Torrent.
Now the MPAA has a "map" of the participants, a graph of all clients of a particular stream. Simply send this as an automated message to the ISP saying "This current graph is bad, block it". All the ISP has to do is put in a set of short lived (10 minute) router ACLs which block all pairs that cross its network, killing all traffic for that torrent on the ISP's network. By continuing to spider the Torrent, the MPAA can find new users as they are added and dropped, updating the map to the ISP in near-real-time."
Although the approach raises a host of problems which he accepts and he would much prefer that ISPs avoided getting into the copyright policing game, he suspects that this kind of approach provides an attractive cost-benefit picture to the AT&Ts of this world.