Microsoft dropped the ball and is late to the phone and tablet market... Android to me just seems way too cheap to add to devices and there are millions of apps for Android.
What Microsoft would need would be something that can only run on their OS, protected from others from making a similar in demand useful product, and something that will leverage people to have to buy a device with their OS on it to have their product.
One thing that might work is if they made an ARM based Windows OS that was free or low cost, and then make up the money in selling apps, but I wouldnt invest in such a risky investment like this. The market is pretty well saturated with Android and free and other low cost apps.
At this point I see them having a Windows product for ARM processors is just to play along side the big boys in the tablet/smart phone market and not feel left out of it.