Comcast came under fire in April 2008 for throttling BitTorrent traffic on their network, even when network congestion was not an issue. (Daniel A. Begun, The FCC v. Comcast, Round 2 (April 25, 2008) hothardware.com.) BitTorrent peer-to-peer traffic describes distributed download services where a computer requesting a file (often a large file, like a movie) both downloads tiny pieces of the file from multiple users on the internet and provides other downloaders with access to those same pieces. The argument from Comcast was that torrent traffic was all illegal content, such as pirated software and movies, but that is no longer true.
Comcast backed off of its packet content-based throttling plan, but phase 2 is here. "The new system, which is now in place, monitors the amount of downstream traffic a user consumes and not what that traffic is actually composed of." (Daniel A. Begun, Comcast's New Network Throttling Now In Place (January 6, 2009) hothardware.com, via digg.com.)
Comcast would like consumer to believe that this throttling is about protecting its network from bandwidth hogs, like large file downloaders. What is more likely the motivation for this second effort at throttling is the desire to keep its the lucrative video-on-Demand service free from competition created by other download services, like Netflix. This is just more anti-competitive behavior from your friendly neighborhood cable company. Don't forget that comcast also imposes a 250GB monthly cap on users. A high definition movie could consume 5-10GB of capacity in one download. These moves are intended to discourage customers from looking beyond Comcast for video-on-demand. Somebody ought to do something about this behavior.