OS X's kernel (Darwin) is a variant of BSD. I have Mac OS X on my laptop and plan to ensure the engine builds on it before release (its just an issue of finding time to do so) I don't foresee any great problems with doing this tho. Regarding the FBSD port, I wish you luck, I run FreeBSD on one of my server boxes and don't really consider it suitable for my desktop due to hardware support. FreeBSD is excellent as a server platform but I find it lagging behind Linux in the hardware support stakes