While Android may be gaining market share hand over fist, Apple's iOS remains the top revenue-generating operating system for developers in 2011, according to Distimo's year-end report.

Both the Apple App Store for iPad and the Apple App Store for iPhone still beat the Google Android Market in terms of the total revenue generated by the 200 highest-grossing apps. The Apple App store for iPhone generates about four times the revenue that is generated in the Google Android Market.

While Apple’s App Store bested everyone in terms of revenue, Distimo found that the number of downloads in the Apple App Store in the U.S. has been declining for nearly the entire year. The lowest point coincided with the moment when the anticipation for the new iPhone was at its height in September. A rise in downloads occurred immediately following the release of the iPhone 4S in October, and the number of downloads in the Apple App Store for iPhone reached its highest peak this year in November.

Nearly all of the app stores more than doubled their number of available apps in 2011. Windows Phone 7 Marketplace showed the largest relative growth of all stores with more than 400 percent year-on-year growth. When combined, the seven major app stores now offer more than 1 million apps.

Developers depended more on in-app purchases than ever before in 2011. Half of the revenue of the 200 top-grossing apps in the Apple App Store for iPhone is now generated by “freemium” apps. And over at the Android Market, fully 65 percent of the revenue from the top-grossing apps is generated by freemium apps.

According to Distimo’s official count, the Apple App Store for iPhone finished the year at right around 435,000 available apps, while Google’s Android Market topped out just below 350,000 total apps. Users of Microsoft’s Windows Phone 7 (WP7) had approximately 35,000 apps to choose from, while RIM’s BlackBerry App World topped out at approximately 50,000 available applications.

Rovio’s Angry Birds was the year’s top downloaded app across all platforms globally, followed by Facebook and Skype.