There is a great post up on the Pinch Media blog that links to a presentation they did for an IPhone user's group here in New York.
Pinch Media provides analytics for several hundred applications in the iPhone app store. Between them, those applications have over 30,000,000 downloads. In aggregate a really interesting picture is starting to emerge of the app store ecosystem.
If you have any interest in the iPhone applications, as a developer, an advertiser, or an investor, this is a must read.