It'll be their first attempt with any hope of success. I'm thinking Kinect on a tablet, among other things. I don't think people were ready for tablets during Microsoft's initial offerings. Touch interfaces just hadn't come far enough.
"It murders batteries
Devices in the field must be plugged in whenever possible since the Kinect technology drains juice in a hurry..."
Laptops generally have much larger battery capacity (for example my current laptop has 94Wh while an iPad2 has 25Wh), and if Kinect kills laptop battery, forget about mobile for now.
[1] http://www.thedaily.com/page/2012/01/31/013112-tech-kinect-l...
This mindset of "build the OS, the apps will come" is no longer valid anymore, not when competitors like Google, Amazon and Apple are building out destinations and ecosystems, selling consumers on direct apps and tools rather than platforms that promise... especially, in Microsoft's case, when those promises have failed to pan out repeatedly in prior years.