But if you have an ARM based tablet version of Office running on an ARM based Tablet Windows or Phone, why would you even care that the processor isn't X86? M$ is cutting the thread tieing it to the Intel/AMD world. If you can easily cross compile between Metro ARM and Metro X86, then you no longer have a developer/software issue.
The reason M$ failed in the tablet market despite having a 10 year lead was precisely because three people enjoy running desktop apps on a tablet and those three work for Microsoft. Mobile Office will be a step towards correcting that mistake.