if your good at coding, see if you can record users IP address (or facebook connect), and put a button next to the song like "save to list or save to playlist" Just not everyone will (or can) download, having a mobile playlist would be perfect. Just my 2 cents....
I listen for online radio all day. Mixed music, nothing special. But something like that should be really cool, even when lots of people is working from home, from computer. Its a good link.