But will it make a difference?
Somehow I doubt it.
Like Jason mentions in his response, it's more about ARM scaling up, and Microsoft thinking "maybe there'll be some new PC running ARM out, and we'd better be sure Windows will work on that".
I don't see something like a fully featured full sized Windows being released to work on some underpowered ARM based handheld. The point of making Windows actually work on an ARM is all about making sure that if someone comes up with the idea to use an ARM architecture in a full size laptop or something.
And I can definitely see something like an ARM powered laptop in the near future. Other than the x86 family, ARM architecture is pretty much the biggest architecture out there, and it's only been getting bigger and bigger, since there's been pressure to constantly upscale it's capabilities.
I think another factor that's led to this decision by Microsoft is that if there were to be an ARM based laptop or some other device that would normally have run windows (maybe a full fledged desktop??) there's a good chance that if Windows weren't on it, it'd have some flavor of Linux (possibly Chrome OS?), and if that were a huge success, Microsoft would have been so late to the party that they might as well have never shown up in the first place.
