I've found that
http://spoofmac.com works great in Leopard... for spoofing the AirPort MAC address. Not so much for the ethernet. It seems that something changed in Leopard and I haven't seen any information about spoofing wired MAC addresses and success under Leopard.
(A legitimate reason to spoof a MAC address: I'm trying to get two ports in my apartment swapped from the student subnet to the faculty subnet. The net admin has no idea which cables are involved and wants to trace MAC addresses. I've got a couple of routers plugged into the ports... but no way of getting the routers to go through the web forms to register with our student ResNet and therefore no way to get the routers (and their MAC addresses) out of their ResNet jail, and make them traceable so I can get these ports swapped. So... I'm trying to spoof their MAC addresses on a machine that
can go through the web forms to get those MAC addresses out of jail. Unclear if spoofing the MAC addresses on the wireless will suffice. Ay yi yi.)