Why? I'm not hacking the kernel or anything, but I write plenty of systems-level code on Mac OS first and then port to Linux (usually painlessly). It's like any other unix. The only pain I can remember encountering was dealing with a MacOS calling convention weirdness (it uses 16 byte stack pointer alignment even on x86).
A couple of years ago, I created a coin-op internet access system from a stripped down version of Debian, that ran in several hotels near Venice, Italy. It was pretty cool, and would have been difficult to accomplish with anything but Linux (infact, it replaced a windows system that never worked very well). Unfortunately the business side of things didn't work out very well because the guy I was working with was not really on the up and up:-(
I still have the code somewhere, if anyone's interested in doing some kind of deal.