Pinch Media released an awesome presentation about iPhone app usage. I haven't validated their data directly, but if it is accurate it has some very important implications for the companies who think they can make a business with free, ad supported apps.
Check it out:
Most of this is pretty well-known to entertainment and e-commerce people: the hit dynamics, the seasonality, the pricing strategies. But the sharp usage drop-offs suggest that for most apps -- not the highly used, sticky ones, but the vast majority of apps -- too few impressions are being generated to justify an ad model. The key slides are #23-#25.