Say you want your very own copy of Men in Black III today. You can either wait to buy the Blu-Ray DVD, due out in November; rent or buy it online at about the same time from Apple or Amazon; or download it as a BitTorrent torrent today.
A lot of people are doing the latter. According to TorrentFreak, an online publication that tracks where "BitTorrent and copyright collide, "Men in Black III was the most popular downloaded movie last week." (Don't ask me why, it's really not that good!) Of course, if you're one of those folks, you should keep in mind that your IP (Internet Protocol) address is very likely to be recorded and given to the movie's owner.
By whom? We don't know.
Tom Chothia from the University of Birmingham in the U.K., along with his fellow researchers found that multiple groups are tracking the top 100 torrents (PDF Link). They "detected monitors in Top 100 torrents; this implies that copyright enforcement agencies are monitoring only the most popular content on public trackers. Movie and music torrents were most heavily monitored (by 65 and 26 monitors respectively)."
Further, "40% of the monitors that communicated with our clients made their initial connection within three hours of the client joining the swarm; the slowest monitor took 33 hours to make its ﬁrst connection. The average time decreases for torrents appearing higher in the Top 100, implying that enforcement agencies allocate resources according to the popularity of the content they monitor."
This presumes, which seems a safe bet, that these are "enforcement agencies," such as the MPAA and RIAA. Certainly the RIAA has been happy to track down anyone who might be illegally downloading a song for almost a decade. It's probably not the MPAA or RIAA themselves who are actively spying on possible Torrent copyright violators.
TorrentFreak reports that the media companies and consortiums use "companies such as BayTSP and Peer Media…to track the IP-addresses of file-sharers."
No matter who's doing it, the bottom line is that if you want to avoid legal troubles down the road for your video collection you should avoid downloading copyrighted movies and TV shows. While in recent months the MPAA and friends have been targeting MegaUpload and other file-storage/sharing sites and services more so than Pirate Bay users, consider this: those stats showing three hours and the 100 top torrent numbers? They're from 2009.
In 2012, I imagine the anti-pirate forces are faster and more thorough, and eventually they'll use the data collected for something. So, if you really want to be safe, just buy or rent your media. You'll be glad you did in the long run.