Using iAd as default ad network, and AdMob if iAd fails
I've built my app this way and everything seems to be working more or less. After hearing about the notoriously low iAd fill rate I decided that this would be the best method, but I tried googling it and i couldnt find record of anyone else implementing ads in their app like this. Does this violate the TOS?
Answer
Why not use Adwhirl. Its great sdk that enables you to do exactly what you need. You can set priority settings for different ad networks which can be changed on the fly if you find one network performing better than others etc.
It handles all the logic for which ad to show based on request failure or priority without you needing to worry about it. All you do is create an adwhirl view and request an ad. Adwhirl does the rest, including appropriate refreshing. If an iAd fails first time, and then shows an admob, but the next iAd loads successfully, it will be shown instead of the admob, assuming you set iAd as a higher priority network.