Not sure which pieces you're talking about. Code changes to various open source projects within OS X, iOS, and Xcode can be found here (and the referenced sites therein).
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/