Yes and no.
Unix isn't open sourced, and these days is a certification for a family of OSes that match the Unix common standards. There are also still OSes that have a heritage in the original Unix versions - Solaris and some of the BSDs for example. None of these are GNU licensed - most BSD varients are BSD licensed, and some versions of Solaris were under the CDDL
OS X is based off bits of FreeBSD, as well as a micro kernel called L7, and apple has chosen to keep the source of this open - since they have the option to close it off under the BSD license.
Here's all of Apple's open source components.
The Darwin Kernel (based off BSD) is here - while in theory its possible to build an OS off it - see OpenDarwin and PureDarwin. I'd note both projects appear dead, and you would lack many of the desktop components like the desktop and so on.
As such, well, its sort of open sourced, but not in the same way as Linux or BSD. You couldn't build a working system off apple's code, but you could pull in some other things and do so if you didn't mind losing much of the apple UI
Edit - as of feb 2013 there appears to be a new release of puredarwin, which claims fairly major changes I have no idea if this means the project is alive or not. I also became aware that darwin was released in compiled form here, whcih may be a useful starting point for hacking your way around things.