The Open Group has certified Appleis Mac OS X version 10.5 as UNIX. Prior to Leopard, Apple had to describe Mac OS X as UNIX-like or UNIX-based.
It was a minor technical caveat by Apple that was easy to overlook.
In the early days of Mac OS X, 100 percent POSIX compliance was not considered a priority for a primarily consumer-based OS. The variations from standard POSIX APIs was very small, and didnit have any real effect on Mac OS X or its functionality. There were many more important tasks to attend to.
However, over time, it became increasingly important for Apple to have full UNIX certification for Mac OS X, and Leopard will have that distinction.
In some cases, that lack of certification could have had an influence on some government contracts. With some UNIX professionals, it occasionally brought some unfounded suspicion about Mac OS X being somehow inferior, somehow not real UNIX.
However, that will all be behind Apple when Leopard ships, and Apple will be able to claim that Leopard is UNIX. It seems like a small issue in general, but itis been a nagging issue in some very technical circles, best put into the history books at last.