IMHO, this question can be interpreted in two ways. At the literal level, Mac OS X gained UNIX 03 certification with the release of Mac OS X 10.5 (Leopard), and is as Unix as it is possible to be.
On a more subjective level, Mac OS X is merely a
rewrite of replacement for the classic Apple operating system, Mac OS. The interesting aspect of Mac OS X is that it is a POSIX compliant OS that happens to use an XNU kernel, which can trace its roots to BSD. And that it was released as an open-source project, Darwin. The POSIX compliance lets software packages written for Linux or BSD be ported to Mac OS X.
However, Mac OS X is more than just the kernel, and IMHO, Mac OS X is closer in spirit to Mac OS than to any other Unix variant.
Update: Link to Joel Spolsky's take on Biculturalism where he primarily talks of the schism between Windows and Unix cultures, but also touches briefly on why Apple succeeded in providing an excellent "Desktop Unix" with Mac OS X.
Update 2: Link to Unix philosophy as explained on Wikipedia.
"Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface."
My highly subjective opinion that Mac OS X is closer to MacOS than a "traditional" Unix OS stems from my opinion that Mac OS X has usability as its overriding goal, and not the above. That said, I do agree that it is also valid to consider Mac OS X a true Unix by focusing on the many points of commonality.