Pretty sure this wasn't in Jaguar. Go to AudioMIDI setup (in /Applications -> Utilities) and choose "Properties For" and select your audio interface. If you have one that supports multiple pairs of outputs (say a MOTU audio interface like a motu 828), this will give you more options on the right under "Audio Output." Press the "Configure Speakers" button which should no longer be grayed out. You will notice that you can select several different speaker configurations: 5.1, 6.1, 7.1 surround as well as quadraphonic, hexoganal, and octoganal speaker settings. Very cool! You can even send a test sound to any channel.
What would be nice is if DVD player could send the surround sound signal to the discreet channels. I know the new player supports digital out from the G5's optical s/pdif.
Anyway, just a neat feature built into the OS that i hope applications start taking advantage of...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031030191049783