OH SNAP ... NOW I GET IT!

Microsoft's strategy for Windows 8 is to bring forth two different operating systems.
The first Windows 8 will be built for the ARM platform and it will support new programs made for Windows 8.
The second Windows 8 will be built for the x86 platform and it will support new programs made for Windows 8 and it will crudely support Windows 7 and Vista programs (and perhaps Windows XP, exclusive to the Ultimate version).

(this is not like Windows and Windows CE, since Windows CE software were unsupported on Windows)

If someone developed a Windows 8 software using the tools Microsoft will make available, then the produced software will work on both ARM and x86 version.
If someone developed a Windows 7 software using the Windows 7 tools, it will work on Windows 7 and Windows 8 (x86 version).

Do you see it, Microsoft can create a cross-platform architecture and keep the legacy codes, it just needs a sneaky tactic like the one mentioned above.
I would be very surprised if this is not the tactic they will be employing.