If we're going to be technical, since they don't run a PC BIOS (they use the newer EFI), they're not compatible with the original IBM PC.
There is basically no difference between a modern Mac and a PC - except for Mac OS X, which can be run on "Hackintoshes".
Which is beside the pedantic point, that to be an "IBM PC compatible" you have to NOT run EFI.
I never said that PCs are called PCs because they ARE IBM PC compatible, only said they are called PCs because in the past "IBM PC compatible" defined their category.