Lucas Gonze notes a couple of posts regarding the need for and barriers to universal song IDs.

Lucas points out:
the problem is not so much technical as it is economic. No major content provider has an incentive to use anybody else’s song IDs. Maybe if there was a huge installed base of playlists that used Musicbrainz song IDs or iTunes IDs then it would make sense for Rhapsody to resolve these IDs to their own catalog, but until that point Rhapsody would be unilaterally disarming by allowing a third party to define the namespace.

He is bang on. This is also why I am not certain that Music DNS will take the day. However, there is a group with a vested interest for universal song IDs: the music labels (and artists). And they have a vested interest in ensuring that a third party does not control the space. It's not just music recommendation systems that would benefit, but all added value content, UGC, and data services related to that song require a universal ID if an economy around those songs is to develop.

The problem is I am not sure labels understand this yet, but they will eventually.