Afraid of excessive bandwidth usage, ISPs have been throttling BitTorrent traffic for almost two years now. While most ISPs simply limit the available bandwidth for BitTorrent, Comcast has now taken it one step further, and is actively preventing its customers from seeding, if the latest user reports are to be believed. Comcast is not alone in this: Canadian ISPs Cogeco and Rogers use similar methods on a smaller scale. These more aggressive throttling methods can't be circumvented by simply enabling encryption in a BitTorrent client, as Comcast is reportedly using an application from Sandvine to throttle BitTorrent traffic; the application apparently breaks every (seed) connection with new peers after a few seconds if it's not a Comcast user, making it virtually impossible to seed a file, especially in small swarms without any Comcast users. Although some users report that they can still connect to a few peers, most of the Comcast customers chiming in have seen a significant drop in upload speed.
Sandvine's throttling works like this: a few seconds after one connects to a peer in the BitTorrent swarm, Sandvine sends a peer reset message (RST flag) and the upload immediately stops. Most vulnerable are users in a relatively small swarm where one only has a few peers to which to upload. However, only seeding seems to be prevented, as most users are able to upload to others while the download is still going; once the download is finished, though, the upload speed drops to 0.