According to http://torrentfreak.com/comcast-throttles-bittorrent-traffic-seeding-impossible/ Comcast's blocking affects connections to non-Comcast users. This means that they're trying to manage their upstream connections, not the local loop.
For Comcast's own position, see http://bits.blogs.nytimes.com/2007/10/22/comcast-were-delaying-not-blocking-bittorrent-traffic/
