Amongst all this talk of swarming RSS-linked downloads with BitTorrent, I'm amazed no-one (well, one person) has mentioned Konspire, which was designed to carry out exactly this kind of task. The only real difference is that whereas clients poll the server to find out if an RSS feed has been updated, file servers send out pings to clients over the konspire network when new files are ready.
There's probably a good case for each of these systems, at particular levels of scale and channel popularity.