It's actually quite simple and will NOT damage any Apple-supplied applications, nor Apple's Installer.app. Disable the SUID bit on /System -> Library -> CoreServices -> AuthorizationTrampoline. To do this simply type (in the Terminal):
% sudo chmod u-s /System/Library/CoreServices/AuthorizationTrampolineYou will be asked for your admin password, but at least this time you will know what it's being used for!
[robg adds: I'm not sure what effect this might have on the various installers -- if it can't run as root, does it still run? I don't have anything lying around that I can test this with, so until someone comments, I'm not sure of the impact of this change.]

