There are many legitimate reasons why someone would want to spoof a system's MAC address, and I won't go into them here. Long story short: in Tiger, it was extremely easy to spoof the MAC address, as explained in this hint. In Terminal.app, just type:

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 target MAC address. This code cannot be used in Leopard, but it turns out the solution is extremely simple, and just as straightforward. Again in Terminal, just type:

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

That's it; you're done.

[robg adds: Based on the comments to the author's original blog post, it seems this isn't working for everyone on every combination of hardware.]