Steve Jobs, Apple's CEO, has published an essay on Digital Rights Management (DRM) system that Apple uses to sell music on its online music store, iTunes. The songs purchased from iTunes are DRM-protected and only play on Apple's iPods. He makes a neat back-of-the-envelope calculation to estimate the fraction of DRM-protected songs people carry in their iPods:

Through the end of 2006, customers purchased a total of 90 million iPods and 2 billion songs from the iTunes store. On average, that’s 22 songs purchased from the iTunes store for each iPod ever sold.

Today’s most popular iPod holds 1000 songs, and research tells us that the average iPod is nearly full. This means that only 22 out of 1000 songs, or under 3% of the music on the average iPod, is purchased from the iTunes store and protected with a DRM. The remaining 97% of the music is unprotected and playable on any player that can play the open formats.

He argues that 3% is hardly enough to lock people in to iPods and iTunes for their music.

There is also a piece toward the end where he states that the biggest obstacle for doing away with DRM are the four big music companies. This, however, he argues is not rational, since

In 2006, under 2 billion DRM-protected songs were sold worldwide by online stores, while over 20 billion songs were sold completely DRM-free and unprotected on CDs by the music companies themselves.

This means that 10% of the music sold has been DRM-protected. This is 7 percentage points higher than the earlier estimate of the DRM-protected content carried by people in their music players (iPods). This may be a real effect: people carry a larger fraction of DRM-free music than is sold because they share them more easily. But the "share effect" is clearly very limited: less than 10% of the DRM-free music sold seems to have been shared.

One might expect that with online purchases this effect might be bigger due to the greater ease of sharing already digital music and the relative hassle of digitizing audio CDs. But this must only lead to a rather small increase of the "share effect". Even a factor-of-two increase stands no chance of disturbing the profits of the music industry compared to the boost getting rid of DRM would bring to the market.

See also: The phantom menace arguing against the use of "piracy" and "theft" for software (and by extension, any digital content) duplication.