I would think, that with the way a lot of P2P shared files are mislabeled, that they would have to download the file, or have someone download a file and play it to see if it has the content which is in violation of copyright, etc. I think they just target people sharing audio files, get a sample of a few of those audio files, and then sick the lawyers on them.

Some people are considering using Freenet, as a way to distribute MP3 files and be more anonymous than P2P file sharing. [link|http://freenet.sourceforge.net/|http://freenet.sourceforge.net/] [link|http://www.freenetproject.org/|http://www.freenetproject.org/] But I got a feeling the RIAA and others will try and shut that down too.

Then again people share MP3 files on IRC as well. Not to mention web sites.

So it is a lot more than just P2P file sharing, but P2P file sharing seems to be the most popular, so they are hitting it hard. Getting the ISPs to reveal who had the IP address the sharer was using at the time, and filing a lawsuit against them.

Edit: Freenet link typo