TechCrunch has the word that Apple may be implementing a "VIP" ad program within its upcoming iAd service. An email reportedly sent from a Quattro sales representative mentions a "Verification of iTunes Purchase" program, in which an advertiser for a certain app would be able to see his or her users' iTunes purchases, and deliver ads based on that information. Quattro is the mobile ad company purchased by Apple a while back, and presumably the division overseeing iAds.
That verification process would allow app creators to not only know which of their customers has already bought other apps of theirs, but it would enable them to only show you ads for apps you haven't yet purchased. In Flick Baseball Pro, for example, you wouldn't see ads for Flick Fishing if you'd already bought it from the store.
That might give Apple the advantage against other ad programs on the App Store, and even if not, it would let them charge a premium price. Since everyone else is guessing at iTunes sales data, Apple would actually be able to deliver real numbers for iTunes purchases, and help app developers target app ads.