Apple's also posted a preliminary version of its Unix Porting Guide, which makes for some interesting reading for people transitioning from Unix (and Unix-alikes) to Mac OS X.

update: a word of caution is in order here -- this is a preliminary document, and some sections may be so preliminary as to cause more harm than good.

reader Marko Karppinen points out that a few members of the darwin-development mailing list have taken issue with sections of the document:

The document is littered with factual errors and on many occasions it states as fact the exact opposite of the truth. There errors were pointed out to Apple the day the Guide was published, but they've yet to address the deficiencies.

so, caveat emptor.