I can't believe I left this off my prediction list I posted a couple of weeks ago.

The attached excel model can walk you through how I derived my conclusion. Bear in mind that my entire model is inherently underestimating the sales difference between platforms because the spreadsheet models unit sales as a step-wise function instead of a smooth growing curve. Still, you can see that Android will surpass Windows in 2015.

https://docs.google.com/file/d/0B0eqwubTBaPScUZTaC1nNDVJeTQ/edit