It appears that you can easilly change the physical address of an ethernet interface on MacOSX.
This facility is included in ifconfig(8)
For example, I was faced to an issue in NYC, with the cable internet access of the house I live.
I just fire up a xterm and type the following as root.
#ifconfig en0 lladdr 00:40:ca:3d:dd:3d (put your correct MAC address)
then, using DHCP I was able to reach the internet.
NB: This does not work on the wireless interface, I currently do not know why, maybee apple does not want to allow MAC spoofing on the Wireless NIC….?
Anyway, you can still try to patch the kernel source code with etherspoof.
Enjoy it !

UPDATE: The issue was related to the broadcom proprietary driver. Some workarounds exist by binary patching the driver. You have got to inject (in hexadecimal) the MAC address of your choice in the driver and reload it. Follow this link for more information.
A GUI called SpoofMAC has recently made things easier. Great job !