A simple script that anyone could grab and put in the HEAD of their blog/website (or install via a plugin for blogger/wordpress/typepad). Anytime there is an MP3 linked to on the page (and/or a band or song), a little ‘buy now’ icon appears next to the MP3 link (or the artist/band/song name turns into a link, if it isn’t already) and links to where you could buy that song via iTunes or eMusic or Amazon.com. Or maybe a little iTunes/eMusic/or Amazon.com icon (their favicon.ico).
You could extend this in the future so that if there is mention of a band/artist name on a webpage, a link appears next to it, which might open up a layer (like how snap.com gives a site preview) to display tour dates, bio/wiki info, ebay/Stubhub items for sale (using your affil link of course), and links to the band’s official website and myspace page.
Would avid music bloggers install it? Sure — just to provide all this extra available info to people. But Definitely, if they were getting a cut of the action. I don’t think there’s much action involved (isn’t it like $0.05 per song via iTunes that affiliates receive? Although eBay affil earnings are good) — and with affil programs from many vendors, tracking would be a bitch.