As lots of others have noted already, EFF has won the Grokster case in the Court of Appeals.

Summary: if you make truly decentralized P2P software -- like Gnutella -- you can't be held liable for any copyright infringement that takes place on their networks. This is the "Betamax principle," from the famous Supreme Court case that established that Sony wasn't responsible for any infringement that its customers undertook with their VCRs.

The decision paper makes for very interesting reading, inclusive of the simplified history/overview of P2P systems.