In my small mind it is quite simple:
Windows 8 ARM devices are for people who want an alternative to an iPad. Hardware wise I would expect a range of devices ranging from cheap all the way up to exotic.
Windows 8 Non-Arm devices are for everyone who also want a traditional PC type option.
When it hits the market I think that the differing price points between the two hardware platforms will be a great boon in helping "us" understand our own requirements as well as differentiating Windows 8 from previous generations of Windows.
Go on then, tell me it isn't that simple.