Android is gaining traction because of so many devices and tablets currently working and supporting Android - Win7 is a gone case - weak effort for MS to come back. Nokia Symbian is a lost case too. so the only thing left to compete is IOS.
IOS still has traction but it wont be for long because of IOS being closed to other platforms - its all about innovation and brand value - how long that will suffice - its hard to tell - lets see what apple comes up with next.
Android based devices are surely the future. there will be meeGo based devices by intel - all in all it will be ball game ... competition is good - benefits the consumer hahaha.