You mean ARM architecture? Having an ARM-based SoC is not the only requirement to make all ARM-compatible (but <i>not</i> cross-platform) operating systems work on other unintended devices. That's not to say it's impossible (as you can see on the picture above) but it requires a large amount of engineering. Even then, it wouldn't be usable (poor battery life, waste of screen real state, imprecise point input, etc.).
Even if it was possible, however, the only one that would be portable would be Windows Phone 8, which is going to be based on the same Windows 8 kernel and WinRT. I am pretty sure Windows 8 doesn't support low resolutions; that's the area they are trying to tackle with WP8.
I think it's very unrealisitc to think that a program that can run on a regular x86/64 machine, will also run on a ARM-based mobile device.
People don't expect to port their programs from their Macs to their iDevices. I think Microsoft shouldn't be punished just because they also happen to do the same. (The mentality I'm seeing recently is that because they <i>are</i> late, they should have engineered a powerful, power efficient, mobile OS that can do the same as its desktop counterpart. The thing is, those characteristics go against the very reasoning Microsoft used to decide to make the user experience a priority. Emulated x86/64 software on ARM devices are "weak" [due to emulation], and most certainly power inefficient.)
Alternatively, Microsoft said they were going to be very clear when selling/advertising Windows 8 about the fundamental distinctions between the desktop Windows 8, and the ARM.