A study by Anirban Banerjee, Michalis Faloutsos and Laxmi N. Bhuyan ("P2P:Is Big Brother Watching You?") has shown that peer-to-peer file sharing users who do not employ blocklists can be tracked by media companies or their agents.
Here's an excerpt from the paper:
A naive user is practically guaranteed to be tracked: we observe that 100% of our peers run into blocklisted users. In fact, 12% to 17% of all distinct IPs contacted by a peer are blocklisted ranges. Interestingly, a little caution can have significant effect: the top five most prevalent blocklisted IPs contribute to nearly 94% of all blocklisted IPs we ran into. Using this information users can reduce their chances of being tracked to just about 1%.
Source: Anderson, Nate. "P2P Researchers: Use a Blocklist or You Will Be Tracked. . . 100% of the Time." Ars Technica, 10 October 2007.