Microsoft are going into this very half-arsedly. They've got one of their own boys running Nokia, for one thing, but instead of instructing him to maintain the alliance Nokia had with Meego and sweet-talk them into supporting WP as their push to get x86 onto mobile they basically dumped Intel cold and now Ottelini has run straight into Android's arms. A unified desktop/tablet/phone OS with the ability to cross-run applications without the need to recompile them for the ARM instruction set would have made Microsoft into the monopolistic behemoth they were back in the 90s.
I suppose Intel's chips just aren't ready to power mobile devices, so they elected to turn out some ARM tablets rather than stay out of the race altogether, which is sensible, but I still think they should be doing more. They and Apple are the only companies who have their OSs on all 3 form factors, but they're the ones that made the least effort to integrate their devices together, treating them like islands rather than as a seamless experience. RIM and HP/Palm made a lot more effort to help people treat their phones and tablets as a single contiguous work surface. (Though RIM failed because they just weren't, though now that they're putting QNX onto phones too they could give it another go, and HP failed cause the whole thing was kinda half-baked and they quit the race before they were barely past the starting line)