A very different, developer centric model is PurpleTalk, which offers an ad exchange model. Basically, when apps are launched, splash screens feature a banner ad for another app, encouraging cross trial. Here's how they explain it:
The AdShare program developed by PurpleTalk is the most cost effective way to promote your iPhone Apps. PurpleTalk partners that participate in AdShare will include a small PurpleTalk module into the applications they’ve developed. Then, each time a user launches an application that has this PurpleTalk module, a brief splash screen appears that promotes a different application on an embedded banner. Through these splash screen promotions, users will become aware of many different applications, including yours.
There's very little other info on their site in terms of revenue model. Models like this are fairly common in the blogger world, where apps promote one blog on another, with a ratio of 9 exposures on other blogs for every ten on yours. That enables the company to sell exposures on the remaining inventory. I tried one, and got lots of exposures, though relatively few clicks. But that experience may well be very different because of the more focused viewing area on an iPhone and the fact that the ads appear on a splash screen versus in what (on my blog at least) is a rather busy sidebar. With this model you are certainly reaching an audience predisposed to downlaoding apps. I mean, they already did by definition!
I don't know if that is their model. Their pages explain that developer do not earn money for the ads but rather points that entitle them to ad exposures on other apps.
Thanks for reading, and don't forget to write.
