Originally Posted by Npl
This is based on misunderstanding I believe, there already is a "Windows for ARM" and its running on those Windows phones. Its called WindowsCE and been around since 1996, its quite different to the desktop variant though.
Nope. MS did unveil a beta version of "normal" NT-based Windows running on ARM (along with a beta port of MS Office to NT for ARM)
Windows NT was designed to be cross-platform from the very beginning. Early (3.x) versions of NT supported FOUR platforms (x86, PowerPC, MIPS and Alpha) and even through the whole NT 4.0 cycle, Alpha was fully supported. Windows 2000 would have been released for Alpha as well, but Alpha CPUs were discontinued right before the Alpha version was to have been released. Thus, it's not too shocking to see NT running on ARM.
Anyway, as for gilgamesh's question, that's actually what most people using ARM are doing at the moment. Still, with the smartphone and tablet market growing the way it is, MS would be silly to not have a horse in the race...
Also, @alexh, RiscOS 5 has been ported to Cortex-A8 based systems like the Beagleboard, though the lack of a 26-bit mode on the chip means that some really old programs won't run. Given that the hardest work of porting the OS to a 32-bit only design has already been done (by a small group of unpaid hobbyists, no less), adapting that port to nVidia's "super ARM" platform shouldn't be that hard, it's just not being done as AFAIK, there is no sort of Beagleboard-style hobbyist-targeted motherboard using Tegra available, which means there's no suitable hardware for such a port.