Watchlisted — they have an amazing hacker potential that could make a dint in the triade of Last.fm, Rdio and Spotify, what can I say for HypeMachine, Exfm, 8tracks and all the other youngsters...
There still isn't much to go on with speculation. It is safe to assume though Twitter won't be going near music-distribution — their tap into the existing infrastructure of Digital Music is a great example of "new way of building", which is frankly: "not inventing the bicycle all over again".