IT consulting firm Gartner is even more bullish on Android than IDC. The company says that by 2015 that almost 50% of the world's smartphones will be Android devices. This is partly because of the "open OS" and because of greater pricing flexibility with Android (vs. the iPhone).
Also like IDC Gartner expects Windows Phones' share to exceed iOS by 2015:
Here's what the company says about Apple, RIM and Windows:
Provocative and well-reasoned though they may be these predictions are unlikely to play out exactly as forecast.
The mobile market is so dynamic that current trends cannot be expected to continue as they are today. Specifically though Nokia has bet the farm on Windows Phones (and they're good devices) it's unlikely that the marriage without more -- like more improvements, apps and aggressive marketing/pricing -- will boost Windows Phones' share to the anticipated levels.
Android will run into major regulatory and anti-trust problems if its dramatic gains continue to the point where 50% of all smartphones run the Google OS (as smartphones gain more share relative to feature phones). It will be the Europeans before the Americans that would lead the charge by forcing Google to make it a truly open platform independent of Google's authority and control.
Finally I believe that Apple is likely to try and address the "lower end" of the market in some way with an entry level or "nano" device.