I don't like the fact that DP ads only support Adsense as its alternative display. Adsense banned me, so I can't use it. I am using Chitika (which has been horrible) and Infolinks (also horrible). I would like to at least display Chitika while no one has bought DP ads, but then display DP ads when someone has bought them, the same way you can do with Adsense.
Yeah, just make a webpage with nothing but the Chitika ads on it, and use the URL of that page as your fallback URL.