My prediction is that someone is going to make an open-source music browser where your money goes directly to the artist.
The users will be essentially purchasing a service, they'll buy the song and it'll be stored online for them, easily downloaded with no software required. That song will be yours forever. It should be able to be downloaded super fast, a smart p2p that can detect files in your direct vicinity to get it to you faster. It should have the option to play it online if you don't want to d/l it. You should be able to browse through other people's libraries and even listen to what they're listening to at the same time.
It should be a smart, intuitive browser, and when you buy rights to a song you should get anything you want from that song, lyrics, rock band tracks, tablature, videos, etc. It'll essentially be a bundle that the artist puts together, but you can pick and choose whatever you want to d/l, and the rest sits there to either be viewed online or downloaded later at your convenience.
The reason behind all these features is so that your actually paying for something that you can't get for free.
The storage, convenience, speedy d/l and cheap price that mostly goes to the artist.
Whoever develops the application gets a small cut to keep things organized.
The key would be in the device itself. The purchases that you make should have the option to show up on your phone bill. The browser is online and it should offer services like free radio.
If you hear a song you like on the radio, you should be able to click on it, and have the option to purchase it.
By being super cheap, like a quarter a song, it'll be worth it.