Here in Germany they go after BitTorrent by prosecuting the seeders. They get the IP address of the seeder then go to the ISP to find out the owner of that IP address (linked to an address). They then send a letter in the mail to the owner of that Internet contract telling them that they will be fined for illegally sharing copyrighted content.

My former landlord received one such letter because "one" of his tenants, who shall remain nameless, was seeding. The fine was somewhere around 900 euros each for 2 different movies. Fighting it involves getting a lawyer and trying to get the fine down.

When I was home visiting my mother she received a nastygram e-mail and phone call from Comcast because "one" of her sons, who shall remain nameless, just happened to have BitTorrent running with some seeded files. Of course, in the US there is no enforcement behind such notices so I told her to ignore it.