So, I’m now using OS X Server for a machine which does some email processing, but for Various Diverse Reasons, it is beneficial to me to be able to use the spamc client directly, which means I need to have spamd running. Luckily, someone else cleverly already solved this problem, producing a launchd .plist file for spamd.
Convenient, and it works nicely. Should be applicable on any reasonably recent OS X Server; I’m using it on 10.6.something.