Each device that connects to a network has to have a globally unique MAC address. If only one arduino is present in the network it makes no difference what the MAC address is, as long as this address does not belong to another device, which is super highly unlikely.
So if I make a direct connection between two arduinos with ethernet shields and use the tcp/ip protocol, communication between the two won't work if both have the same MAC sddress? Where in this direct connection setup would the MAC address of the two arduino ethernet shields checked? I've ordered a mega with an ethernet shield to do some arduino to arduino testing, but just curious about the actual useage of the MAC address.