One thing people seem to be misunderstanding in the whole Win8 discussion is that MS isn’t really trying to shoehorn regular Windows onto tablets ala the iPad. The “tablets” in question that will be running full Windows 8 with full legacy app support are touch-enabled x86-based systems – things like the existing HP and Lenovo Win 7 tablets (or touch-based desktops like the HP TouchSmarts). Presumably, one could build keyboardless x86 tablets, but you won’t likely see many.
For portable tablets, MS is developing a parallel version of Windows that runs on ARM processors (the lightweight chips powering the tablets on the market). Since apps running in the new interface are built on web code, they’ll work either on ARM or x86 systems. ARM tablets will not support legacy Windows apps. They may have some sort of “native code” app support, but MS has indicated nothing about how it’ll work or what it’ll look it.
MS isn’t developing a single OS that will be distributed across everything. They’re developing a single interface that’s designed to work with touch as well as traditional input and will be used on all the versions of Windows 8.