The real foundation for cross-platform code -- from mobile platform to mobile platform -- is whether WP 8 will support OpenGL ES 2.0. The explosion of quality Android games came courtesy of the NDK and ES 2.0, allowing for an almost direct migration of apps from iOS with only minimal I/O changes.

Given that the mobile GPUs are targeted to that, I have to think it will. Having portability between the desktop and mobile is less valuable, really, given that even the most impressive mobile GPU pales compares to a miserable desktop GPU. The limitations would render such an exercise much more prohibitive.