It appears that Comcast has stepped up their fight against BitTorrent by outright cutting off users' transfers and blocking their ability to seed downloads.

ISPs have been limiting the available bandwidth for BitTorrent traffic for almost two years now, according to TorrentFreak, and though BitTorrent protocol encryption seems to work against most forms of traffic shaping, Comcast's more aggressive throttling methods can't be circumvented just as easy.

It is reported that Comcast is using an application from Sandvine to throttle BitTorrent traffic. Sandvine breaks every (seed) connection with new peers after a few seconds if it's not a Comcast user. This makes it virtually impossible to seed a file, especially in small swarms without any Comcast users.
ISPs have long argued that bandwidth-hungry P2P applications can cripple their network and thus some controls are due in order to make the experience good for all users. However, customers aren't likely to agree that limiting their internet connections is an acceptable solution.

TorrentFreak says setting up a secure connection through VPN or over SSH seems to be the only workarounds for Comcast's throttling methods.