Well... Apple didn't want people running Windows on their machines before... but new hardware changed that didn't it?
What about CPU sockets? Are they the same regardless of platform? Say you went out and got a dual 2.66GHz Mac Pro and it just wasn't enough power for you... could you go to a place like Newegg and order two, 3.0GHz "woodcrest" Xeon processors, open up your Mac Pro and replace the CPU's? (obviously, this wouldn't be the most cost effective thing to do... but could you do it if you wanted to?)
Yes, everything is identical except they removed the BIOS and inserted an EFI chip, or whatever. Processors, videocards, RAM, harddrives, optical drives, they are all cross platform interchangable.