I see it as something like the historical gold rush - developers flocked to the app store because they sensed there was a lot of money to be made. This vastly increased the supply of apps, which in turn decreased the demand per app, and so decreased revenue per developer. I think there will inevitably be a fall in the rate of apps being added to the app store, and sucesses will continue to be focused on a handful of publishers.

PS: This is probably wrong in some fundamental way (I'm not an economist)!