BitTorrent is effectively "unstampable" for two reasons.
The first and largest is that the RIAA/MPAA have only been able to stamp out Napster/Morpheus, et al because they have shown in court that the primary purpose of those networks is to infringe copyrights. BitTorrent, on the other hand, has been around for quite a while and was simply designed as an efficient tool to move large files across the Internet without overloading a single server. It has a huge number of legitimate users and is heavily utilized in the Open Source community. It's true that people DO use it to infringe copyright, but the RIAA/MPAA will never be able to show that infringement is its primary purpose. Without that, their cases will fail.
The other reson any lawsuits will fail is the lack of any real NETWORK. Unlike Napster, Gnutella, and the other file swapping applications, BitTorrent doesn't let you search for MP3's or video files across the network. You have to locate a download on your own and download the torrent file, which will allow you to connect to people sharing THAT FILE ONLY. It's not an efficient way to locate pirated films and music, and doesn't qualify as "file sharing software" under any of the currently accepted legal definitions.
If anything, the RIAA/MPAA will have an easier time controlling BitTorrent downloads than with previous file sharing tools. Because BitTorrent doesn't allow people to share their file lists or search for specific movies, BitTorrent pirates have to rely on websites and chatrooms to locate the torrents they want to connect to. Killing piracy simply requires them to shut down the websites directing people to the pirated material...something already covered by current law that they don't need a lawsuit to accomplish. If they find a webpage with illegal torrents, they just need to C&D the websites host and shut it down. That'll keep BitTorrent piracy out of the mainstream while preserving it for legitimate users.
Good points. I think the lack of a real network will be overcome if BT really takes off due to the "network effect." If only a few people are sharing a particular file, then it is'nt much use to anybody (kind of like being the only person in the world owning a FAX machine). BUT, if millions of users begin using it 24x7, then the utility of BT grows exponentially and induces more users. The problem is finding the torrents you want to join at any point in time. If you use a web site or chatroom, then these are easy targets for RIAA to prosecute.