The ability of iPhone apps to generate insane revenue is pretty well-documented. It's not limited to Apple's App Store, either; the Android Market is capable of making some serious cash as well. But the App Store's greater popularity and much larger userbase make it a unique case.
Burstly has the story of Air Horn, a self-admittedly "gimmicky" app (it does exactly what you think it does) made by Simpaddico. Air Horn is a free app with in-app advertising, and for reasons at which I wouldn't care to guess, it became the number one best seller in the App Store for several days. It had been out for more than six months before hitting it big, and had zero advertising budget. How did it happen? Burstly shows some of the numbers behind it.
It took just over 188,000 daily downloads to hit number one, and retained that top spot even after dipping to under 130,000. In the eight days measured here, which include the app's ascent to the top of the charts as well as a several-day stay at number one, the app served about eight million total ad impressions. Half of those eight million were for third-party advertisers, and half were for a sister app called Annoying Sounds. Annoying Sounds, despite being "not even a top 1000 app" before Air Horn hit it big, jumped all the way to number five on the strength of its sister app.
In terms of advertising revenue, Burstly isn't listing exact numbers, saying only that the app netted over $20,000 in eight days. Part of that is due to ad optimization; as Air Horn's downloads went up, the value of each CPM (cost per thousand) went up (a total increase of 25%).