As I type this, I'm building-up a Darwin kernel so I can get IPv6 and IPsec working w/ Mac OS X. Building OpenBSD, developing on it, and using it, I've become extremely spoiled. Darwin's kernel build is a shitstorm by comparison.
The info I found was written by itojun, in KAME's latest newsletter:
The instructions aren't perfect, but I've managed to get my kernel up w/ IPv6 support, as well as build some of the user-land tools. There's some header madness, abandon all hope yee who enter.