All we need is to type this in Terminal
sudo ifconfig en0 lladdr 00:00:00:00:00:01
sudo ifconfig en0 ether 00:00:00:00:00:01
Just change 00:00:00:00:00:01 with the MAC address we need.
It seems that for older versions of Mac OS X (prior to Tiger – OS X 10.4) we will need this patch: http://slagheap.net/etherspoof/ .