If you're thinking purely in terms of 'smartphones' whenever you think of iOS, Android and Windows Phone, you've blown it. It's so incredibly short sighted to think of these OSes as a smartphone play - they are all so much more than that.
These are the three OSes going to power consumer devices (phones, tablets, laptops, desktops, TVs, etc) for the next 20-30 years, or be the template for such. Google recently bought Motorola, so many assume they'll be making smartphones and tablets under a Google brand. But a key thing being overlooked is that Motorola's smartphones are also PCs; starting with the Atrix, every flagship Motorola phone has had the capability to turn into a laptop or desktop PC, or media center with the requisite dock. You drop the phone in the dock, start using the keyboard and mouse on your big screen LCD, and you have an instant computer - one that was in your pocket seconds earlier.
This is as much a part of Google's strategy as devices; Google is poised to take on the 'PC' market, and Android is finally their vehicle to let them do this.
With Windows Phone, many posit that Microsoft is moving everything to the Windows RT kernel (aka Windows 8); the phone could be as much Windows 8 as the tablet or desktop. Microsoft have been pretty candid about their duelling Metro/Desktop environments in Windows 8, but it starts to paint a familiar picture once all the information is in play. Your Windows Phone, which may run on ARM or perhaps x86, could dock and power a desktop display, with a mouse, keyboard, and even legacy Windows apps. In the future, I see no distinction between Windows Phone and Windows; for Windows Phone to fail, the entirety of Windows has to go down with it.
A whole new generation of computing, where everything can be powered by one device; that's where Microsoft and Google are positioning themselves. They are platform makers, this is the most obvious and inevitable possible platform play. If you extrapolate a little, it's quite clear that this goes beyond devices; if we're talking about dumb desktop shells that you dock into, why not the same for phones and tablets? What if the computer itself is something you always wear - a wristband perhaps. Wirelessly, it could beam its display to a blank shell of a smartphone in your pocket, or the blank shell of a tablet, or a desktop PC, or augmented reality glasses.
Of course, this won't happen tomorrow, but it could easily happen in the next five years. It's not something on the distant horizon, either. We will start to see more docking phones like Motorola's, or phone/tablet devices like Padfone, or Google's Glasses technology. Eventually the physical connectors will go away, and the docking will all be wireless - at that point, for how long will we still need distinct devices, CPUs, memory and data connections for each display and input mechanism in our lives?
Apple, unlike the other companies, is a device maker, and completely opaque. I'm not sure a single-device strategy would make sense for them, at least in the short term, but I do expect iOS to spread to more devices and form factors. We've already seen it with iPad and AppleTV, and I still feel that iOS and OS X are on a collision course for the desktop. All of these OSes are the mainstream personal computer platforms of tomorrow.
The personal computer hasn't yet come. One day we will be the personal computer. Current generation smartphone OSes are only the tip of the iceberg, and the big players understand this. It's a completely different game being played now. And this, more than anything else, is why companies who think they're just making smartphones (RIM, for example) will have no place in the world of tomorrow.