Not quite true... the optical drives and primary hard drives are Apple certified and marked so on their labels. The Mother Board is also proprietary. Joe can get close to, but not exactly, the same parts.
My point is that you can build a PC with the same guts, not necessarily the same proprietary BIOS, ROMS or what have you which really is the difference in hardware between Apple and PC's.
http://www.osx86project.org/
They play around with OSX on x86 pc's with different hardware.