Which is why you
are going to have problems running Windows on Macs.
Apple not only has no reason to make Macs compatible with Windows, they have an interest in making Macs incompatible with Windows.
If Windows can be run on a Mac with no problems, then getting Mac OS X to run on a Windows compatible PC isn't going to be that hard either.
Besides, the processor is
not the deciding factor in compatibility.
And I can give you plenty of examples where this is the case... like NeXT and Apple computers of the early 90's. Both NeXT and Apple were using Motorola's 68040 processor in their systems. Could NEXTSTEP run on a Quadra? No. Could System 7 run on a NeXTstation? No.
PC makers go out of their way to be Windows compatible. Which is understandable as that is their primary OS. But Apple has no such restrictions on it as they make their own OS.
As for getting Windows to run on a Mac, I would guess that someone is going to need to design either a boot loader (based on Darwin) to help run Windows... or someone will design a PCI card that supplies the additional help Windows would need.
I highly doubt that Windows is going to install on an unmodified Intel based Mac when they finally ship.