If you are considering or using an ad-funded model, you might like to take a look at mAdserve the new open source Ad Server for publishers. The iOS and Android SDK together with open source php server code allows you to host your own ad server connecting to over 31 global and local ad networks.
Hosting your own service means you aren’t paying a middleman for aggregation and won’t be beholden to changes in terms of service. The only concern I have is scalability. However, mAdserve claim you can achieve 100 Million monthly ad-requests on a low-end machine which is adequate for the long tail of apps.