From the same whacko (me) who actually spent the time telling you in detail how to set up a customized mount point system under OSX comes another rather long-winded tail of how to set up SpamAssassin Milter under OS X.
As with all of my tips, this is NOT for the faint at heart. Compilation of three applications is required, so the Developer Tools must be installed. Symlinking of Apple-installed binaries. Things like that ... be brave.
Oh, and if I may pander to the Fink Developers out there, please consider making packages for this!
[robg adds: SpamAssasin Milter is a plug-in for sendmail that routes all incoming mail through the SpamAssassin spam filter. As Numbski notes, this is not for the beginners, though it sounds very very useful (coming from someone who gets about 80 pieces of spam a day). I hope to find the time to work through the setup in the near future, but for now, consider it untested.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030312162624758