No idea but I don't think display ads would be that great since it's mostly passive software. I would think there are better ways to monetize it - premium enhancements, amazon/itunes referrals, cloud backup/streaming your library, even just selling it for a buck ... 'best' trumps 'free' a lot of the time on mobile.
I swear I was reading a different HN thread on this topic yesterday or even today, and the very idea of paying for a music service was considered extremely offensive.
I pay for rdio and SubSonic. Granted my subsonic license is grandfathered in from before the developer changed to a subscription model, but the point remains.