Now, I've read through it. Grokster and StreamCast got slammed in the decision- but not P2P in general. In fact, in the opening paragraphs, Justice Souter makes it very clear that P2P technologies, themselves, have substantial non-infringing uses.
They, very carefully, applied this decision to this specific case, and here's there logic. While there's no way on a P2P network to know what your users are doing, a simple search turns up a vast quantity of copywritten works. The court found that Grokster knew about the piracy in a way that made them legally culpable. More important than that, they actively encouraged piracy (especially in the case of Streamcast), and sold their product with the promise of piracy.
So what does this mean? Now, I'm no legal expert, but here's what I got out of it- systems like Bittorrent won't have this issue. At the very least, the people who make clients are not at any risk- because there is absolutely no way they can know what you do with that client. It'd be like suing a calculator maker because someone used it to embezzle. Trackers, however, are at risk. That's nothing new of course, they have been the entire time. Users of course, are still at risk.
But, in my reading of this decision, I don't see anything that is going to hamper honest, non-infringing use. Further, if a company doesn't maintain the network, they have no legal culpability at the moment. I'll specifically quote the first paragraph:
The question is under what circumstances the distribu-
tor of a product capable of both lawful and unlawful use is
liable for acts of copyright infringement by third parties
using the product. We hold that one who distributes a
device with the object of promoting its use to infringe
copyright, as shown by clear expression or other affirma-
tive steps taken to foster infringement, is liable for the
resulting acts of infringement by third parties.