Fred von Lohmann has published an interesting article on the intricacies of law surrounding the development of P2P software entitled: What Peer-to-Peer Developers Need to Know about Copyright Law. It is a good article which tries to clearly describe what direct, contributory and vicarious liability means, and how developers can potentially avoid legal pitfalls in the development of these kinds of P2P software. Given the findings in the Grokster case, it would behoove potential authors of P2P software to read and understand each of the points raised. In particular, he believes that the Betamax defense (that it is enough to show that there are substantial non-infringing uses) is under concerted attack (like the INDUCE act) and in his words:

In short, the law surrounding the Betamax defense remains in flux, putting P2P developers (and all technologists) on unpredictable legal ground.

In the end, the article concludes with 10 bits of advice for potential P2P developers.

These are steps you can take that may: (1) reduce the chance that your project will be an easy, inviting target for copyright owners; and (2) minimize the chances that your case will become the next legal precedent that content owners can use to threaten future innovators.

Worth reading, especially if you are interested in developing P2P applications.