Well, functional enough for me to FTP (in both directions) a couple of DVD images at in excess of 10MB (yes, mega bytes) per second.
modprobe eth1394 is the key.
The interfaces on your Firewire card will then appear - mine turned up as eth1 and eth2.
This is a very cheap, fast way of doing very local (i.e. the length of a Firewire cable) networking.