David Byrne has a journal entry musing on the Spiral Frog iTunes challenger. He thinks a Google like search-based, ad-supported distributed pay-for-download system is probably in the works.
My ideal model would be a simple web-shopping engine that bands could run from their own sites, where fans can download a DRM-free mp3 directly, after paying 49 cents or whatever. Why have a middleman at all, be it Google, Advertisers, iTunes, labels etc? You'd need softwarde that generates a one-time dynamic link so the customer can get the mp3 but not mail or post the link elsewhere. Most long-tail indie bands/groups would have a fairly low-volume need for bandwidth.
Music fans do a pretty good job of finding stuff they like - reading blogs, magazines, Pandora, internet/sat radio, etc. They should just be able to go to the band's website, push a paypal button, pay a few cents, and get a one-time download link. No DRM or advertising required. This should be easy. If it exists I'd love to find out about it.