On the other hand, for the i386 copies of FreeBSD (which the great-grandparent was talking about), Apple does release working copies of their FreeBSD-derived kernel.
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/
Fair enough, although the parent of that post just talks in general about larger projects (such as the toolchain) and the post I directly responded to talks about Xcode.
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/