There was an article in a local newspaper last month about this. They ( the music industry and its attorneys) are going after people who "offer copywrited material" for download on P2P networks, they also got a judge to give them a court order to force ISP providers to provide lists of people that use the P2P software and thier IP addresses.
Since they have tried unsucessfully to thwart the file sharing by going after the networks ( I think everyone remembers the publicity that Napster got a while back), they figure if they start scaring people by making them pay dammages it will make them stop, and its perfectly legal.
Also they are using the same software that you are using (P2P) to catch you by running searches on copywrited songs and taking names and IP addresses.
The only way you can defeat that is to "unshare" your main music file so it cant be accessed by the network through a search. Problem there is that if everyone does that it does exactly what the music industry wants, pretty much cuts out the file traffic .
Another way they can get you is using the "Hotlist" option that some networks offer, where you can choose someone , add them to the list and browse all of the shared files they have on thier machine.

They (the music industry and the lawyers) are out for blood and they are here to stay so Id watch out and maybe lay low for a while to see how this thing comes out, even if they fined you $10.00 per song it could get pretty expensive,especially for those that have a couple thousand song files on thier machine,not to mention the lawyers fees to defend you.
Plus it IS criminal so it becomes part of your record, and will show up if anyone accesses the criminal databanks, if you are found guilty of an offense.