Doesn't this already exist in the form of iTunes? Actually, what's interesting is that Tila Tequila, the most popular MySpacer is launching her music career directly through iTunes. It looks like MySpace paid music is already in the works, and having it get launched sometime this year wouldn't surprise me at all.
iTunes and Myspace are going to be the two areas where musicians can make any money off of their productions. Everything else just doesn't have as large of an audience of people who are passionate about music, and willing to put their money where their mouth is. There'll most likely be a pretty active and thriving niche offerings like Will.I.Am's, but the majority will stick with those two large brands. And that's cool, because they're both pretty decent services. Myspace's doesn't actually exist, but Myspace is great for musicians, and adding paid downloadable tracks is a very natural and easy extension.
I was actually just talking to a guy in an indy band the other day, and he mentioned to me that a large amount of money made on music doesn't even come from record sales anymore, the majority is off of the tour. I don't know how true that is, but it really would be interesting, and would actually confirm some thoughts I came to independently.