My colleague Fred von Lohmann has revised and condensed his seminal white-paper in which he explains to P2P developers what the law actually says about P2P systems and how to design your technology to minimize your chances of getting (successfully) sued.
In the wake of recent decisions on indirect copyright liability, it appears that copyright law has foisted a binary choice on P2P developers: either build a system that allows for thorough monitoring and control over user activities, or build one that makes such monitoring and control completely impossible.
Contributory infringement requires that you have “knowledge” of, and “materially contribute” to, someone else’s infringing activity. In most cases, it will be difficult to avoid “material contribution”–after all, if your system adds any value to the user experience, a court may conclude that you have “materially contributed” to any infringing user activities.
(Thanks, Fred!)