Might be worth a shot if someone is feeling adventurous. From what I've read, the standard to carry FireWire data connections over an RJ45 connection require certain newer hardware in the RJ45 port controller on the motherboard and some support in the OS. No evidence yet that either exist on the new MacBooks and OS X 10.5
It theoretically can be done ... just read about it in wikipedia. but the port has to allow it physically , not the case here according to my understanding.
The adapter referenced by TMA's post above is for a cable tester. It is not ethernet. It is just adapting the ethernet port on a wire/cable tester to the connectors for firewire cables so the tester can be used to verify that a firewire cable is electrically sound.
I would be interested in finding if an actual firewire to ethernet external box actually exists... The other clue that that adapter is bogus is that it doesn't have an external power supply. Ethernet ports do not normally supply power (yeah I know there is something called Power over Ethernet, but only specialty networking equipment actually supports Power over Ethernet -- it is not standard in the typical cheap network hubs or switches you see in the store.)