This is cool in the very geekiest sense of “cool”. From Ars Technica:
One of the big draws of OS X has always been the UNIX-like, BSD heritage of the operating system. Apple has always touted OS X as UNIX-based and played up the security, stability, and compatibility that comes with the BSD foundation. The company has also gotten in some trouble with The Open Group over Apple’s use of the UNIX name, when in fact OS X wasn’t actually UNIX-certified. All that is changing, though, since the upcoming Leopard release has received the UNIX 03 certification (PDF) as of May 18, meaning that Mac OS X 10.5 on the Intel platform is a “true” UNIX OS, rather than just being UNIX-like.
It may all sound like semantics, but the certification is actually quite important for Apple and for the OS. The UNIX 03 certification means that Leopard conforms to the Single UNIX Specification Version 3 (SUS), a specification for how things like the shell, compiler, C APIs, and so on should work. Of course, the UNIX 03 certification is only for Intel-based Macs, but I suspect that has more to do with Apple not bothering to get OS X certified for an older architecture. This latest news marks Leopard as the first BSD-based OS to receive the UNIX 03 certification, which is quite an impressive feat, and also adds Apple to a very short list of official UNIX 03 OS vendors (IBM, Sun, and HP being the others).
The fact that Apple can use the UNIX name more freely is cool, but the certification will have a far bigger impact on enterprise customers. Any software written for the SUS specification is easily portable to a UNIX 03 operating system, meaning that enterprise customers who need a “real” UNIX for their applications can now use Leopard servers if they so desire. Leopard’s certification also gives developers another option for a development platforms, which could translate into some extra Mac sales. Things like the GUI may not be portable, but any code written according to the specification should be. This would make the UNIX 03 certification a big draw for developers and enterprise customers alike, by providing another (perhaps cheaper) option for developing and running UNIX applications.