substantial noninfringing uses

Is peer-to-peer file sharing software illegal, in and of itself? If you're going to ask yourself that question, you need to keep in mind at least a couple of things. One of them is that the software can be used legally by people to share files that aren't copyright protected, and it has been.

The other is that the software can do this from one computer directly to another without using a centralized network.

Why does that make a difference? Well, when you're being sued for contributory copyright infringment after releasing software that can be used in substantial noninfringing ways, and you don't have knowledge of nor control the connection between people who might be using it to infringe upon someone else copyright, it might. At least, that's what a federal judge in Los Angeles appears to believe.