Great idea. I don't have an Iphone, so can't use it, but thought of the benefits of such a concept before. Consider marketing it as a web app, monthly fee, to coffee houses/bars/restaurants.
And, more efficient would be to tie into a Pandora/Music Genome system, where patrons can vote on genre. The more songs available, the less likely you'll have even two people vote for any one song. But if you link each song to genre...
Or make it like a jukebox and take micro-payments to have your choice of song played. Want your song higher up in the queue, pay a little more. Not sure how this works in a commercial environment though (royalties??).
And, more efficient would be to tie into a Pandora/Music Genome system, where patrons can vote on genre. The more songs available, the less likely you'll have even two people vote for any one song. But if you link each song to genre...