ThisIsMyNext, quoted Microsoft at its Worldwide Partner Conference as saying:
Redmond would eventually have just a “single ecosystem” for PCs, phones, tablets, and even the TV. “You can have full PC compute power available in whatever form factor you like,” said Lees, noting that the Windows on ARM demo motherboards were smaller than mobile phones. “We won’t have an ecosystem for PCs, and one for phones, and one for tablets — they’ll all come together.” Lees went on to say that Microsoft wants to provide “coherence and consistency” across different devices, “particularly with Xbox,” and that the goal isn’t just to share UI, but also core technologies like Internet Explorer.
It went on to break down just what that implies:
[...] Microsoft’s next next OS will run on PCs, tablets, phones, and the next-gen Xbox when it hits sometime in 2015 / 2016.