By way of a slashdot post, there's an interesting thought-blurb on Comcast's BitTorrent filtering:
http://www.cnet.com/8301-13739_1-9769645-46.html Essentially, Comcast detects that BitTorrent's in use, and sends RST packets (messages that essentially states, "please terminate the connection") impersonating the BitTorrent user.
However, it's illegal to impersonate others... so.... hopefully someone will drag them to court. Preferably in Maryland or in Illinois. If the laws exist here.