ifconfig en0 lladdr 02:01:02:03:04:05
ifconfig en0 ether 02:01:02:03:04:05
This must be done as the superuser and only works for the computer's ethernet card. Instructions on spoofing AirPort Extreme (2.0) cards are available here. For OS X 10.5, instructions are available here. There are not, as of yet, any known ways to spoof original AirPort (1.0) cards.
This page on the KisMAC wiki (see section "MAC Address Spoofing") contains links to download several utilities to change your MAC address on Mac OS X, including MacDaddy, SpoofMAC, and AirportClown: