Visible says Android has passed Apple with developers because Google's developers kit costs less, and because Google has done a good job marketing its open source model. Further, Android phones are selling well now, so there's plenty of customers.
The gap between Apple and Android isn't that great, so Apple should hold its lead in apps for a while to come. And, most developers say they are developing for more than one platform at a time.
Other than Apple and Android, the rest of the mobile platforms are being left behind. Vision Mobile says, "anecdotal developer testimonials suggest that half ofPhone MVP developers (valued for their commitment to the platform) carry an iPhone, and would think twice before re-investing in Windows Phone."
Do Android developers outnumber iPhone developers? This Vision Mobile poll indicates that more mobile application developers have experience with Google’s Android platform than with any other mobile platform, including Apple’s iOS. Developers polled cited: large market penetration, speed of development, potential to make money, and low cost dev tools as reasons for initially choosing Android.