Tip #367   MAC Address Spoofing on OS X

If you need to change your wireless MAC address (I had to do this when I used another computer to try to fix mine on a network with MAC filtering) this command comes in handy.

Pre 10.5(I know this works in Tiger but it should work 10.3 up):
sudo ifconfig en0 ether 00:00:00:00:00:0

sudo ifconfig en0 lladdr 00:00:00:00:00:00

just switch 00:00:00:00:00 with whatever mac address you want to pretend to be.