[pmjordan]

The xnu kernel is pretty far from FreeBSD. It's based on CMU Mach 2.5 (mainly virtual memory and IPC), 4.3BSD (mainly networking and VFS) and the Apple/NeXTSTEP-developed device driver framework called I/O Kit (which is written in C++). I don't think any kernel changes have been merged back into BSD. At this point they've probably deviated too far anyway.

Building xnu from public sources is quite an undertaking. It does still seem to be possible (although it got a lot harder with the x86 transition due to more proprietary drivers). The PureDarwin distribution still seems to be based on the Snow Leopard kernel, I'm not sure why. http://www.puredarwin.org/