In MGM v. Grokster, now pending before the U.S. Supreme Court, all parties have made the assumption that most P2P file transfers infringe copyrights. Two theories contradict that assumption: a significant number of individuals who transfer files over P2P networks may have a license to do so, and the Copyright Act itself may exempt the transfer of certain categories of entertainment files over P2P networks from the definition of infringement.
Yours For Keeps: MGM v. Grokster, 23 J. Marshall J. Computer & Info. L. 209 (2005)