I have looked at all the free/ad sponsored solutions that keep popping up and having used a couple of them I think that rather than struggle to find a way to give people mp3s for free and wonder how to integrate the ads, perhaps it is time to junk the promotional single and replace it with the v-single ? Don't get me wrong, the iTunes single will still be an option as will a physical but delivered directly from your website ?

With the video you can incorporate a sponsor, ads etc and know that it would be seen. Would be ironic in a way if this is picked up on and video becomes becomes the initial cash cow rather than a loss leader.