Mac OS X is Unix
The question and debate about whether Mac OS X is truly Unix or not was debated online a long time ago on slashdot.com and osopinion.com. The reason for the debate was the fact that the name Unix belongs to the Open Group (http://www.unix-systems.org/). Here's the url to the concluding article on osopinion concluding that OS X was Unix-like though not true Unix and therefore not recognized by the Open Group: http://www.osopinion.com/perl/story/9451.html. It should be noted that a footnote at the very bottom of the article contains a link to the Open Group web site. After hearing about the question of Apple's OS X position in relation to other unices, the Open Group updated their web site to include Apple as a company supporting the unified Unix specification:
So, as far as the Open Group is concerned, OS X is as Unix as any other flavor out there. Just because OS X is different in its installation than the type of Unix you use does not make it not Unix. What Unix is is determined by the body that controls the use of the name Unix, which is the Open Group.