Considering the Google Play Store is now having about 2.5 billion app downloads per month (announced I/O), while the iOS app store only 2 billion per month, and the fact that there have been 48 billion app downloads for Android and 50 billion app downloads for iOS so far, then some simple math shows us that Android will be closing the gap within 4 months, and then will go on to become the most popular app platform for developers, by October.
This, of course, is only the beginning. Android has been activated on 900 million devices so far (not counting the ones that aren't licensed/approved by Google), and Google dreams of activating most of the 5-6 billion smartphones that will exist in the future on the entire planet.
The revenue per app is still not as big as the revenue per app on iOS, but the difference is pretty small at this point, and Google has been constantly optimizing its Play store with this goal in mind lately. As Android distances itself from iOS in terms of total units on the market, the overall revenue per app per developer should also increase, making Android the highest paying platform, too, if nothing else at least thanks to the sheer number of Android devices.
But as I said, I do believe Android will catch-up to iOS in revenue per number eventually, especially as more people start using Google Wallet. I still think the slow adoption of Google's payment methods has been one of the biggest obstacles in getting people with Android devices to pay for apps. Why bother buying an app, when you don't have an account set-up, when you can just tap the install button of a free app instead?
Apple benefitted from the tens of millions of iTunes subscribers, who had their credit cards tied to the iTunes accounts, from the early iPod days, so it was very easy to get these people to tap-and-buy apps ever since the app store was launched for iPhones. Google only managed to begin to get people to make Wallet accounts with Android 4.0+, and ever since they've done that, Android revenue per app has also increased a lot. When you make it very easy for people to pay for content, they do it. When most people will be having Google Wallet accounts, it should get much easier for developers to monetize their apps on Android.