@jerv: what does FreeBSD have to do with it? The FreeBSD license, and BSD licenses in general, don’t do much to restrict what you do with the code. They basically just expect you to credit the original owner and respect his copyright, and propagate a warranty disclaimer. It’s not like the GPL, a license that says modified code has to remain open. You can use BSD code in proprietary products pretty much without hindrance.
But, yeah, legality is probably a non-issue. The question of why anyone would want to use a piece of shit like MacOS X in the first place aside, your chances of getting caught are pretty damn near nil and, if you have a license to the software to begin with, there is probably little Apple can legally do about it – ‘cause it’s not like installing their OS hurts Apple somehow. The only real concern you have is being caught pirating, if you go that route.