How did they know I downloaded ‘Meet the Fockers’ on BitTorrent?

Slate: CSI: BitTorrent. How’d They Know I Downloaded ‘Meet the Fockers’? MP3 podcast here. Daniel Engber explains how corporate snoops often employ outside contractors — and even hackers — to do the work. They use software to determine the Internet protocol address of the users downloading (and often simultaneously uploading) movies and music. They also “seed” file-sharing networks with material to trap users into downloading it and then catch them (NY Times synopsis).

Excerpt from the Slate piece:

Some file-sharers claim that media-security firms also seed networks with dummy torrents to trick potential pirates into revealing themselves. (One file-sharing company says the MPAA hired a hacker to steal its trade secrets.) A couple of years ago, a pair of young hackers started circulating their own fake downloads that displayed a "Bad Pirate!" message and broadcast your IP address when you opened them up. ...