After several maddening hours of trying to get Redmine working on Snow Leopard, I finally realized that it was trying to access the system-wide Ruby/Rails install in /System/Library/Frameworks rather than the one I had installed via MacPorts.
I know you can add to the $PATH for _www by editing /System/Library/LaunchDaemons/org.apache.httpd.plist, but does this stop it from looking in Frameworks and/or is there a better way to do this?