If you were hoping for some sort of Rosetta-like emulation layer to enable Windows 8 to run x86 applications when installed on an ARM system, well, prepare to be disappointed. EE Times quotes Renee James, the head of Intel's Software and Services business, as saying the ARM version of Windows 8 "will not be running legacy apps not now or ever."
The x86 version of Windows 8, on the other hand, will reportedly have a "Windows 7 compatibility mode." That means, going forward, software companies that want their Windows apps to run on ARM machines might be forced to release new versions compiled specifically for the architecture. Such a requirement wouldn't rule out the possibility of fat executables containing machine code for both x86 and ARM systems, however.
Then again, that might just be the tip of the iceberg. EE Times says Intel CEO Paul Otellini went so far as to claim, "The ARM guys are getting a port to Windows, but it's really four ports [because] every OS has to be written to a chip so Microsoft is really doing four ports of Windows to ARM." Otellini didn't elaborate on what exactly those four versions are, though EE Times says he "showed slides suggesting they could be targeted to specific versions of the ARM core or SoC implementations of the cores from ARM licensees."
I suspect we'll soon hear some sort of clarification from Microsoft about all this. You've got to remember Intel has quite a big stake in seeing Windows on ARM fail, so seeing the chipmaker spread a little fear, uncertainty, and doubt around the subject seems natural.