Is the Mac a PC 2584
If it's not native, it's not a clone, it's an emulator.
The compatibility test is about how well the hardware is a clone of the original, but...
Is the Mac a PC 2585
Mike True. But Apple is doing an excellent job with its current offerings. I'd love to be able to put together an OS X box from...
... I agree that it is not relevant for modern systems since there is no one reference design that is being copied.
But you're missing the critical point that Apple providing the CSM *is* a BIOS layer for Macintoshes, hence making its PC offerings analogs of the clones of the 80s.
In understanding what a clone is, we need to define what the reference is. For EFI Macs without CSM or other compatibility with legacy hardware, the reference would be cutting-edge Intel hardware, and since Apple is an Intel OEM it's impossible for their hardware to be "clones" of an Intel reference, since they *are* Intel reference designs.
Since longhorn will be EFI bootable, CSM-less Macs would be some sort of "Wintel" clone even without BIOS compatibility, but not proper "PC Compatibles", since PC compatibillity implies legacy BIOS compatibility (not to mention audio, graphics, and USB compatibility).