A few days after I criticized Sony BMG for missing the point of DRM-free music--it's about convenience, which isn't served by forcing customers to walk into stores and buy cards and redeem them online--they proved me wrong by agreeing to release their catalog for sale on Amazon's DRM-free MP3 store. That means you'll be able to buy and download just about any song from Amazon and play it on any software or device. Let's hear it for universal playback, a mere ten years after the first MP3 player went on sale.
Subscription services like Rhapsody and Microsoft's Zune Pass will probably hold out for a little while before agreeing to limit download numbers (like eMusic), and Apple might need to go back to the negotiating table with the labels to secure DRM-free AAC files from all of them.
Now comes the real question: will Amazon (or anybody else) be able to mount a challenge to iTunes, or will integration between iTunes and the iPod/iPhone/iWhatever trump the convenience of universal playback for downloads? There's room for innovation, as iTunes has some gaps--music recommendations and community participation stand out--but competitors will have to be careful to avoid unnecessary complexity if they hope to fill these gaps. That's been the appeal of iTunes since the beginning: simplicity.