Apple vs. Anti-DRM
Apple appears to have taken an active technological stance against the recent emergence of iTunes Digital Rights Stripping applications. Two such applications have been able to effectively strip the Digital Rights Management from Protected iTunes Songs to allow them to play on unauthorized machines and MP3 players.
While Apple's legal team has been effective in shutting down the sites that host some of these applications, Apple's most recent iTunes revision (4.5) appears to furthur thwart these existing applications.
Appleinsider notes error messages when attempting to de-protect the latest files with Fairplay.