The Recording Industry Association of America have recently won a lawsuit against Verizon, an American ISP. Under this, Verizon were forced to provide the RIAA with the personal details of four subscribers suspected of using Kazaa to share copyrighted materials.
However, I doubt this will be the death knell for file sharing networks. It is expected that the successors to Kazaa and its brethren will be able to hide their IP addresses, thereby protecting the anonymity of those whose use them. More details and some thoughts on the case by media expert Bill Thompson can be found here.