The "UNIX-ness" of the MAC OS X that everyone is talking about is Darwin
, which is based on the BSD Mach kernel. It is, however, implemented for the Motorola-based architecture used in Macs.
There have been ports of many x86-based *NIX apps, including XFree86 (XDarwin) to OS X, but not all have been ported. The last time I used OS X, it used tch as its shell. I've heard that bash will run as well, but I haven't tried yet.