Well technically MAC addresses can be spoofed rather easily, plus your MAC address isn't available to anything
past your router or your modem, once the PlayStation 3 passes through one of these all you can pickup is the MAC address of the router or modem so Sony could never use that for banning, it has to be the serial number or some other identifying feature of the console they use. But still definitely keep track of your box and if you can't write down that serial number in a safe place.