In order to fight what it deems as “network congestion”, Comcast is developing a way to slow down particularly heavy downloaders as opposed to all bittorrent users:

“Comcast is starting to test new approaches to protecting its network from what it describes as congestion caused by a handful of customers who use far far more bandwidth than everyone else.

It will test new devices that will keep track of Comcast users and assemble a blacklist of heavy users. Those on the blacklist will find that all of their online activities may slow down at peak times: from downloading movies to checking e-mail.” –nytimes bits blog