One of the first things I did (as any Linux developer would) was to install darwin ports. I noticed some interesting things in there. A few that I needed (git) and a few that completely surprised me (dpkg and apt).
One thing that was missing was a Linux cross-compiler. So I did what any self-respecting Linux developer on a Mac would do: I built one.
Don't get too excited. I've only built one worthy of compiling a kernel (which means no C library, no userspace, etc).
The result of my work is here (built on 10.6.3):
You may notice the extra elf.h file, which is needed in /usr/include/elf.h for some programs in the kernel to compile natively on the host (e.g. modpost). The gcc and binutils will unpack in /opt/local/.
In order to cross-compile, you will need to add a few things to your kernel make command line:
make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu- ...
You may notice, like I did, scripts/genksyms/parse.c has a #include for malloc.h which is not on Darwin. You may safely delete that line.
Note that you must already have /opt/local/bin in your PATH. Using ARCH=i386 will also work and compile 32-bit kernels. One last point, the sources for gcc/binutils came from Ubuntu's Jaunty.