The Debian pegasos1-installer (3.3MB) should be able to tell you.

Just download it, put it on your boot image partition and boot it from open firmware. Go through the language selections, once you've done that, it'll auto-configure the network. Now press alt-f2, then enter to activate the console, here type "ifconfig" and you can see the mac address (HWaddr for eth0).

[ Edited by Golem on 2005/6/14 22:56 ]