No surprise that after the RIAA started filing lawsuits against people they allege were distributing lots of copyrighted files, a movement has sprung up to build filesharing networks where the user hosting data can't be traced so easily.
Today, on Kazaa, all they need to do is try to find a file, look at what a user is sharing and try to download it. That gives them the IP address of the party in question.
The suits will push people into systems that don't make that information easily available. One common design being pushed involves removing the peer to peer aspect that made these systems so efficient and capable of distributing files. Namely the connections are no longer direct, the data flows between one or more intermediaries.
In this case, they can request a file but the data will come from an intermediary. Since that intermediary won't log what they pass on (they are just a router) you would have to have a live wiretap on the intermediary to find where the data came from, and that may be another intermediary. You would need live wiretaps on half the net to actually track somebody. The intermediaries have no idea what data they are routing, and are no more guilty of copyright infringement than UUNET is for owning routers.
But this is of course terribly inefficient, especially since the intermediaries are mostly at network endpoints.
There are designs which protect the privacy of users, but don't let the RIAA sue the hosting system. One was the Mojo Nation project, which died, but has spun off technologies like HiveCache and MNet.
In Mojo Nation, files were broken up into many blocks, with some redundancy. For example a file might have 8 different component blocks, any 4 of which can resassemble the file. Those 8 blocks would themselves be replicated all over the net. You could find out what IP sent you a block, but the owner of that IP address would not have any idea what was in it, it's just an encrypted black box to them, so they are not liable. At best you could order them to delete the block after showing that it's part of a copyrighted file using a DMCA takedown. But it's not practical to do.
At least it's P2P. It's sad that the RIAA's crusade will cause people to modify P2P networks into non-P2P, and gain the RIAA nothing.