I love music 2.0 mashups... I've written about a bunch. Most started with someone just pulling together a couple of sources of information to solve a specific personal need. The next logical step? Turn it into a business.
I've been lucky enough to see a handful of new ones that have yet to launch (and some that have). Some are move evolved than others... but the basic concept is the same. Build a music destination without actually having any content themselves. YouTube, Flickr, Last.fm... these companies get the users to provide the content (and/or usage data). Then these tertiary aggregators suck in all the content via open APIs, stitch them together, scatter some ads around the dynamically created pages, and voila... instant business. Most of the companies that are doing this now are small startups. Some haven't even launched, but are in acquisition talks with major media companies.
What a great business model. The fact that they are leveraging freely available/syndicatable content to fulfill an music consumption experience is smart. No licensing costs to them - no legal liability for the "gray" content that they surface. Of course, the labels are going to figure this out and start to try and figure out how to get a piece of the action.