Other hardware developers like HTC and Samsung have developed their own skins on top of whatever Android version they're running. Only the iPhone doesn't allow for such a thing because, well, only Apple uses iOS.
But what if Apple and Google allows apps that have UI based on the Metro concept? Don't say it can't happen. First, Google allows just about anything to go into Google Play. And more recently, Apple approved Tapose, an app, from former Courier chief before Microsoft killed it. J Allard funded Tapose and the rest is history (still ongoing as version 2 is in the works).
And Tapose is almost a self-contained app with muliple mini apps within it (just don't call them widgets or else Apple will reject them).
Today, I came across this app from called Track 8 (iDowloadBlog)
It's pretty close to the Metro-UI. Not quite but there's promise in what Microsoft did. Now, supposed someone came up with an app that agregrates social updates from Foursquare, Google+, Tumblr, Twitter as well as e-mails and IM, you've got something sweet happening all without having to abandon the iPhone or Android.
In fact, I'm certain we'll see more of this because Apple and Google are just not going to be able to hold down Microsoft for too long. However, just maybe mobile apps Metro UI on rival platforms might be enough to kill it.