Mark Rasch wrote an article on SecurityFocus which talks about why its technically improper to refer to file swapping as "theft" and how it is sometimes within the bounds of copyright law to burn copies of songs (or other works protected by copyright law).

A number of years ago, the U.S. Supreme Court dealt with a man named Dowling, who sold "pirated" Elvis Presley recordings, and was prosecuted for the Interstate Transportation of Stolen Property. The Supremes did not condone his actions, but did make it clear that it was not "theft" -- but technically "infringement" of the copyright of the Presley estate, and therefore copyright law, and not anti-theft statutes, had to be invoked.
Here is the full article: "Copying is Theft ..." And other legal myths in the looming battle over peer-to-peer