Researchers from Harvard Business School and the University of North Carolina released a study today that suggests that illegal downloading of songs via P2P networks is not having a significant effect on legitimate music sales and in many cases may help album sales. The Washington Post has the story.
A few choice paragraphs:

Songs that were heavily downloaded showed no measurable drop in sales, the researchers found after tracking sales of 680 albums over the course of 17 weeks in the second half of 2002. Matching that data with activity on the OpenNap file-sharing network, they concluded that file sharing actually increases CD sales for hot albums that sell more than 600,000 copies. For every 150 downloads of a song from those albums, sales increase by a copy, the researchers found.
“Consumption of music increases dramatically with the introduction of file sharing, but not everybody who likes to listen to music was a music customer before, so it’s very important to separate the two,” said Felix Oberholzer-Gee, an associate professor at Harvard Business School and one of the authors of the study.
Oberholzer-Gee and his colleague, University of North Carolina’s Koleman Strumpf, also said that their “most pessimistic” statistical model showed that illegal file sharing would have accounted for only 2 million fewer compact discs sales in 2002, whereas CD sales declined by 139 million units between 2000 and 2002.
“From a statistical point of view, what this means is that there is no effect between downloading and sales,” said Oberholzer-Gee.