I’d just like to interject for a moment. What you’re refering to as Mac OS X, is in fact, BSD/OS X, or as I’ve recently taken to calling it, Mac plus BSD. Mac OS X is not an operating system unto itself, but rather another free component of a fully functioning BSD system made useful by the BSD corelibs, shell utilities and vital system components comprising a full OS as defined by POSIX.
Many computer users run a modified version of the BSD system every day, without realizing it. Through a peculiar turn of events, the version of BSD which is widely used today is often called “OS X”, and ma…