A pair of new Electronic Frontier Foundation reports prove that Comcast is degrading and interfering with BitTorrent, and shows how you can use free software to test your own ISP to see if it is doing the same:
In addition to providing evidence of network interference, the EFF study also explains how Comcast's selective degradation of BitTorrent traffic undermines future Internet innovation. "The Internet has enabled a cascade of innovations precisely because any programmer–whether employed by a huge corporation, a startup, or tinkering at home for fun–has been able to create new protocols and applications that operate over TCP/IP, without having to obtain permission from anyone," the EFF wrote. "Comcast's recent moves threaten to create a situation in which innovators may need to obtain permission and assistance from an ISP in order to guarantee that their protocols will operate correctly. By arbitrarily using RST packets in a manner at odds with TCP/IP standards, Comcast threatens to Balkanize the open standards that are the foundation of the Internet."
The EFF also published a second report (PDF), which provides detailed technical instructions explaining how to use Wireshark to reproduce their study and test for ISP packet injection.
How the AP busted Comcast for blocking BitTorrent
Why Comcast's BitTorrent-fux0r is bad for quality of service
Comcast also screwing with Gnutella and Lotus Notes (!?!)
Comcast actively blocks P2P traffic
Modest proposal for Comcast's net-filtering