The Anti-Circumvention Clause in the DMCA says "No person shall manufacture, import, offer to the public, provide, or otherwise traffic in any technology [...] is primarily designed or produced for the purpose of circumventing a technological measure that effectively controls access to a work protected under this title [or] has only limited commercially significant purpose or use other than to circumvent a technological measure that effectively controls access to a work protected under this title"
It seems difficult to argue that this would apply, because what would the "technological measure to control access" be that a bittorrent client (or tracker) is circumventing?
I also don't know of any precedent where bittorrent software/client itself was ruled illegal (but am not a lawyer).
