Slashback: Circumvention, AOLandfill, Scoffing
MACs are supposed to be unique. With most network devices (not just Xboxen) you can force the device to use a specific address that you make up (possibly causing a problem if the MAC is already in use in your broadcast domain) but real use of this ability is pretty rare.
In any event the first few bits or a MAC are assigned per manufacturer (many manufacturers have more than one) and then they are supposed to go through the rest of the bits assigning a unique set to each device they make.
At least that's how it is supposed to be.