So apparently, not only is Comcast throttling BitTorrent traffic (as I proved with a little experiment a few weeks back), but they are forging your traffic to do it! They actually modify your individual TCP packets as they travel between your computer and their intended destination. This is the same technique China uses for the Great Firewall of China to prevent citizens from accessing certain material on the Internet deemed "sensitive".

As Slashdot seems to always have the most informative and well educated commenter's, I will post what HiThere wrote with regards to the legal implications this may have for Comcast:

If the ISPs filter based on torrent source, then they cease to be common carriers, and lose common carrier protection. Then they immediately become liable for every case of copyright infringement that they are accessory to.

I don't think they'd like that choice.

If they are common carriers, then they are supposed to be indifferent to WHAT they are carrying, like the mail or the phones. If an extortion threat is transmitted by mail, you can't sue the post office. Not just because it's acting as an agent of the govt, but because it's a common carrier. (UPS is just as protected.) They aren't supposed to know or care what they're carrying. If they did, and demonstrated the capability of filtering it by filtering some of it, then they would lose their common carrier status, and become liable as accessories to extortion, e.g.

OTOH, I don't want them pretending to be me. Not at all. That should be grounds for a suit. It should also be grounds for criminal prosecution not only of those who implemented it, but of all of their supervisors, managers, etc. also. Including the boards of directors. It shouldn't have a particular onerous penalty...say 10 days for each separate offense. Cumulative. I'll be generous, and say 1 day per instance. I.e., 1 day per false packet.

Seriously, if you have a choice, please consider switching to another ISP!

Update: Since I'm going to continue following news related to this topic, I will post relevant news articles I come across below.

Comcast is also Jamming Gnutella (and Lotus Notes?)

Forged reset packets are normally the kind of thing that would only be present if a hacker was attacking your computer, but in this case, it's the ISP you pay money to each month that is sending them.

Comcast filtering Lotus Notes (Update)

I finally have an end-to-end trace to share which shows that Comcast is filtering the port 1352 traffic. The images below show that Comcast is impersonating and using man-in-the-middle tactics to filter the traffic as stated in the CNet post.

Not only is Comcast filtering P2P traffic, it's also toying with other traffic! What a surprise!

Comcast Cheating On Bandwidth Testing?

Upon further investigation, it appears that Comcast is delivering this bandwidth only for a few seconds after any new request and it is immediately throttled down. Doing a download and upload test using a significantly large file (100+ MB) yields results more in line with everyday usage experience, usually about 1.2 Mbps down and about 250 Kbps up (but it varies).

The comments on this Slashdot article are really great. Lots of people offer explanations as to what Comcast is doing.

Comcast gets US FCC notice on Web traffic blocking

Comcast, the largest U.S. cable television operator and the second largest high speed Internet provider with more than 11 million subscribers, has repeatedly refuted allegations it blocks certain Internet traffic or applications.

The company said it used bandwidth technology on its network that can slow the delivery of files, but it would not block them outright.

BitTorrent Plugin Detects ISPs Raping Your Torrents

A plugin for Azureus allows you to voluntarily submit reports of possible sabotage by your ISP.

Comcast Makes Nice with BitTorrent

BitTorrent, Inc. maybe. The BitTorrent protocol? I doubt it.