Am I missing something? If you have the music in your iTunes library but not on your phone at that moment wouldn't you just wait until you get back to your computer to sync the music you wanted? Would you buy a song or album that you already own? I don't see how Apple would be losing money with an app like this...
I was thinking more of the lines of a sling box like app. The orb app on the jailbroken device can also stream you live tv from your coputer. Apple wouldnt want that.