I can't wait for Windows 8 ARM. What will happen when there is a version of Windows that can't run old software? Who will be blamed- software makers, Intel, MS? At what rate will the faces of business users melt when their new tablets don't work with some ancient Active X intranet program their company depends on? How will a MS Windows succeed when it will have LESS native apps than the competition off the bat?
What company would ever use tablets for their employees' primary computers?
At any rate, there will still be plenty of apps available. Anything open-source (Firefox, VLC, emulators, etc.) will likely be ready well before the official Windows 8 launch. .Net programs should be compatible with ARM and x86. And of course, everything in the app store will be able to run on both platforms. Finally, while MS hasn't announced anything yet, I'd imagine that you will be able to run WP7 apps on Windows 8.
Anyway, if backwards compatibility is so important for your tablet

rolleyes

, get an Atom-based one. Atom tablets might be where the real excitement is, now that Intel is putting Atom on the fast track.