Of course this is essentially the same service that PressPlay have been offering for some time. They use DRM protected WMA, Apple use their custom DRM to protect AAC. Only difference is you can download and play WMA DRM files on most modern portables, where as you can download and play Custom-DRM-AAC on... Mac iPod (right now anyway).

I believe PressPlay's catalogue is significantly larger, in that they have many indies as well as the big five.

Rob

Disclaimer: I've had a couple of nice dinners with the PressPlay guys, so I'm probably biased. I forget who paid though.