Well sorry to blow out your dreams but Windows 8 ARM wont isnt coming 'til 2013 and it WONT be available as OEM or retail disc in any way.
It will be embedded into the slabs, just like Android.
But since it is NOT open source, you just will NOT be able to add it afterwords.
THERE WILL BE NO WINDOWS ON THE GTAB 10.1. EVER.
Dual-Booting a Windows 8 ARM tablet with Android might be possible, but not the opposite.
That being said; just wait to see the price of the Windows-based slabs; itll be a good 200$ over the comparable Android-based slab.
Creator of WMOD
(AOSP 2.2.1) for Archos 5 Internet Tablet
How to multi-boot your Archos5it english version
/ french version