Ben Trott (he of Movable Type fame) has created step-by-step set of instructions for installing SpamAssassin under Mac OS X. Thanks, Ben!
UPDATE: you need the latest version of Apple’s Developer Tools (the April 2002 release), which are available from the ADC site.
UPDATE 2: SpamAssassin f’ing rocks! I followed Ben’s instructions EXACTLY on my home machine, and installation went smoothly. I’m assassinating spam as I type this. My usual flood of 15-50 spams per day is about to get dam(n|m)ed!