Microsoft wants you to be able to run any app in all three ecosystems    



From DailyTech:

At the a Financial Analysts meeting, he remarked:

The first of those [beliefs] is that we really should have one silicon interface for all of our devices," he said. "We should have one set of developer APIs on all of our devices. And all of the apps we bring to end users should be available on all of our devices. [We want] to facilitate the creation of a common – a familiar – experience across all of those devices, but a fundamentally tailored and unique experience for each device. [We] should have one set of developer APIs on all of our devices.

Of course it may be impossible to escape some sorts of optimizations for more serious apps; after all Windows Phone is a very different screen size, while Xbox has extra media and gaming hardware resources.

  Microsoft Looks to Unify Development for Xbox, Windows, and Windows Phone