Changing the MAC address of your network interface card can be useful for a number of things. Here is how you change it in both Tiger and Leopard:
For Tiger (10.4.x) open up the Terminal application and run the command:
sudo ifconfig en0 ether 00:00:00:00:00:00
where en0 is the network interface you wish to change the MAC address of, and 00:00:00:00:00:00 is the new MAC address you want the interface to use.
For Leopard (10.5.x) open up the Terminal application and run the command:
sudo ifconfig en0 lladdr 00:00:00:00:00:00