"The most common format of music on an iPod is 'stolen'" - Steve Ballmer.
Most iPod users are walking around with stolen songs on their players, according to Microsoft CEO Steve Ballmer, who is apparently able to discern these things, perhaps by use of some sort of crystal ball or similar device.
As an aside, I'd really be interested to know just what percentage of the mp3s floating around in the world are actually legal. If anyone has any information on this, please let us know.