I ran across this infographic on ReadWriteWeb, which shows a breakdown of mobile operating systems in use by countries across the world. (Click for full size on readwriteweb.netdna-cdn.com.)
What strikes me is how it speaks to knowing your target audience. If you are developing an app to reach the English-speaking world, developing for the BlackBerry, Droid and Apple OS will cover virtually everyone.
But if you are trying to reach developing markets—China, Brazil, India—then you must develop for the Nokia platform, which is virtually unused in the English-speaking world except for a small percentage in Australia, and might consider Samsung as well, another OS that has not penetrated the English-speaking market.
All in all, it is quiet remarkable to see how fragmented the mobile OS market is. And according to Deloitte, the market will remain that way for the near future.
(The image is made available by http://www.icrossing.co.uk/ under a Creative Commons Attribution 3.0 Unported License.)