Windows for Devices.com has an interesting rumor. According to the reporter's tip, Microsoft is to open the source code for the CE operating systems. The license, as such, is supposed to be Microsoft's own "Shared Source". That's not really open source or free software, at all. Even so; it would be a significant move.
I'm not sure of their motive in doing this, though it should be noted that embedded operating systems of any sort often require substantial customization to work the way the developers need it to. That could be the long and the short of MS' motivation because it means a developer can pick and choose the features on the device OS in the interest of functionality, efficiency or speed or any of a number of other reasons.