The bane of Digital Rights Management advocates everywhere, the analog hole is why every music copy protection method available now or in the near future will fail.
Audio copy protection tries to limit what a customer can do with the music provided to them by imposing restrictions on how their digital media behaves - some schemes disallow the playing of files on unauthorized computers while others prevent the copying or ripping of audio files at all, allowing them to be played only on record company approved hardware. Some render music CDs as unreadable on computers at all, a method that has been famously circumvented with a two dollar sharpie and a steady hand. But all of these methods can do nothing to get around the fact that in order to actually hear the music it needs to be transmitted through the air. That digital to analog bridge is the analog hole - no matter how many restrictions are placed on a user's computer, the music still needs to be heard and once the data wings its way to a computer's sound card it's fair game to be copied at will - a patch cable from the card's audio out to audio in ports usually does the trick.
This in and of itself wouldn't be a big deal if each potential sharer had to do the ripping themselves - it's not rocket science but it's certainly not extremely simple, either. The thing that has the record companies in such an uproar at the moment is the internet, peer-to-peer file sharing applications in particular - instead of each copyright violator ripping music in their garage for their close friends, all it takes is one user with an internet connection and all the record companies' efforts go up in smoke.
Some technology pundits put a particularly Orwellian spin on things, envisioning a future where individuals will be required to have an implant installed in their heads that only allows legally purchased music to be heard at all. Given enough time, they'll probably try it. Wouldn't surprise me in the least.