The Mac OS X kernel source (xnu) has been released for Mac OS X 10.6 Snow Leopard: here

Building xnu requires some open source (but not pre-installed) tools. Darwinbuild is the most reliable way for building these dependencies and xnu itself. Until that is ready, you can build the tools manually as follows:

1. Download the build tools source(s)
   
   $ curl -s -O http://www.opensource.apple.com/tarballs/cxxfilt/cxxfilt-9.tar.gz
   $ curl -s -O http://www.opensource.apple.com/tarballs/dtrace/dtrace-78.tar.gz
   $ curl -s -O http://www.opensource.apple.com/tarballs/kext_tools/kext_tools-177.tar.gz
   $ curl -s -O http://www.opensource.apple.com/tarballs/bootstrap_cmds/bootstrap_cmds-72.tar.gz
   
   
2. Unpack the tools
   
   $ tar zxf cxxfilt-9.tar.gz
   $ tar zxf dtrace-78.tar.gz
   $ tar zxf kext_tools-177.tar.gz
   $ tar zxf bootstrap_cmds-72.tar.gz
   
   
3. Build cxxfilt
   
   $ cd cxxfilt-9
   $ mkdir -p obj sym dst
   $ make install RC_ARCHS="i386 x86_64" RC_CFLAGS="-arch i386 -arch x86_64 -pipe" RC_OS=macos RC_RELEASE=SnowLeopard SRCROOT=$PWD OBJROOT=$PWD/obj SYMROOT=$PWD/sym DSTROOT=$PWD/dst
   ...
   $ sudo ditto $PWD/dst/usr/local /usr/local
   Password:
   $ cd ..
   
   
4. Build dtrace
   
   $ cd dtrace-78
   $ mkdir -p obj sym dst
   $ xcodebuild install -target ctfconvert -target ctfdump -target ctfmerge ARCHS="i386 x86_64" SRCROOT=$PWD OBJROOT=$PWD/obj SYMROOT=$PWD/sym DSTROOT=$PWD/dst
   ...
   $ sudo ditto $PWD/dst/usr/local /usr/local
   Password:
   $ cd ..
   
   
5. Build kext_tools
   
   $ cd kext_tools-177
   $ mkdir -p obj sym dst
   $ xcodebuild install -target kextsymboltool -target setsegname ARCHS="i386 x86_64" SRCROOT=$PWD OBJROOT=$PWD/obj SYMROOT=$PWD/sym DSTROOT=$PWD/dst
   ...
   $ sudo ditto $PWD/dst/usr/local /usr/local
   Password:
   $ cd ..
   
   
6. Build bootstrap_cmds
   
   $ cd bootstrap_cmds-72
   $ mkdir -p obj sym dst
   $ make install RC_ARCHS="i386" RC_CFLAGS="-arch i386 -pipe" RC_OS=macos RC_RELEASE=SnowLeopard SRCROOT=$PWD OBJROOT=$PWD/obj SYMROOT=$PWD/sym DSTROOT=$PWD/dst
   ...
   $ sudo ditto $PWD/dst/usr/local /usr/local
   Password:
   $ cd ..
   
   
7. Download the xnu source
   
   $ curl -s -O http://www.opensource.apple.com/tarballs/xnu/xnu-1456.1.26.tar.gz
   
   
8. Unpack xnu
   
   $ tar zxf xnu-1456.1.26.tar.gz
   
   
9. Build xnu
   
   $ cd xnu-1456.1.26
   $ make ARCH_CONFIGS="I386 X86_64" KERNEL_CONFIGS="RELEASE"
   ...
   $ file BUILD/obj/RELEASE_*/mach_kernel
   BUILD/obj/RELEASE_I386/mach_kernel: Mach-O executable i386
   BUILD/obj/RELEASE_X86_64/mach_kernel: Mach-O 64-bit executable x86_64