An interesting view of the growth and the trends of iPhone Apps on U.S. App Store, by Ben Lorica – O’Reilly Radar.
One crude measure for the relative complexity of developing apps across categories is to compare the number of apps per seller. The Top 5 categories in Nov/2009, were Books (17 apps per seller), Travel (6 apps per seller), Education (4 per seller), Reference and Sports (3 per seller). There were also 3 apps per seller in the Games and Entertainment categories in Nov/2009:
(source: Apps Per Seller Across the US iTunes Categories O’Reilly Radar)