A french group have developed an interesting new system. It allows them to monitor, in real time, large groups of torrents and trace them back to their sources..
In a recent test, they monitored 1.2million torrents for 103 days. They found that the content was shared 2 billion times, between 149 million IPs.
The interesting thing is that the system also enables them to track back to the original source of the torrent, and they found that only a handful of IPs were the source for the 1.2 million torrents.
An interesting article on it is at
http://www.theregister.co.uk/2010/04...inuous_spying/
Now, some of the commenters on that article say that it is illegal, so could not be used to gather evidence, but I am not sure it is. They are merely using information that torrent clients actually make freely available.
It could, however, if used by the copyright agencies, be used to track who is leaking the content.