Changing The MAC Address of a NIC in OS X Mountain Lion

This process is also known as spoofing.

# generating a random mac:
openssl rand -hex 6 | sed 's/\(..\)/\1:/g; s/.$//'
# ↑ although this doesn't take care of bit 0 and 1 of byte 0 which should be 0.
#   ( )
# changing the MAC address:
sudo ifconfig en0 ether xx:xx:xx:xx:xx:xx