This is normally done with a tool like ifconfig - though checking on my pre3 it seems to have a crippled version of ifconfig without the option to change the MAC. So assuming there isn't another tool that can do this (and assuming the driver supports it) you'd need to compile a regular version of ifconfig for the pre3.
nate@nate-desktop:~$ ifconfig --help 2>&1 | grep -i hw
[hw <HW> <address>] [metric <NN>] [mtu <NN>]
hw class address
Set the hardware address of this interface, if the device driver supports this operation. The keyword must be followed by the name of the hardware class and the printable ASCII
equivalent of the hardware address. Hardware classes currently supported include ether (Ethernet), ax25 (AMPR AX.25), ARCnet and netrom (AMPR NET/ROM).