oric_dan:
Even an authentic ethernet shield poses some questions in the MAC address world. The first three octants indicate the vendor and an authentic has a sticker that is a MAC address (0xDE:0xAD:0xBE:0xEF:0xFE:0xED) registered to Gheo Sa in Switzerland.
This is in line with my previous comment about lack of support and documentation.
It seems the clone maker of my ethernet shield hasn't registered its products, simply
soldered chips to a cloned board and put it in the mail.
Just remember, if you have a project that requires two or more authentic Arduino Ethernet Shields on the same LAN, the sticker on the back will still have the same MAC, so you are going to have to increment in the sketch them to make them unique. Since these devices are projects/prototypes, the responsibility of registering the MAC would be on the final system manufacturer (you, me, etc) not on the board manufacturer (Arduino, clone maker, etc).
Tom Igoe seems to use a "testing" type MAC in the Twitter sketch of 0x00:0xAA:0xBB:0xCC:0xDE:0x01 that is not registered and ready to increment.