Like the question, like the answer paulwhannel!
I have had this question asked to be a couple of times aswell. Windows (non-NT) users asked this:" OK, so Windows uses DOS, what does Apple use?"
I try to explain that Mac OS is GUI from startup right away, this being the ROM file. ROM goes to RAM >> System boots (wtih extensions, control panels) >> Finder boots. Viola! Mac OS
BTW, Mac OS X doesn't run on Unix. It IS a Unix. What you probably refer to is the Mac OS X GUI (Aqua) which runs on Apple's own implementation of Unix BSD, called Darwin.