Phew! I've just managed to work out how to get my PowerBook on the internet, by connecting over FireWire through my iMac. The iMac's ethernet port is taken (by my girlfriend's PC), and it has no wireless capability, so I needed to work out a simple way to get online. Mac OSX computers can network over a Firewire cable, so I connected a cable between the two, but I was encountering major headaches trying to get on the web. Or even to anything as simple as connecting from one computer to another using SSH or ping.
Here's what you (probably) need to do: on the computer with the internet connection, make sure internet sharing is enabled for "Built-in FireWire" (look in System Preferences > Sharing > Internet). Then go to the Network Preferences and set up your Firewire network connection as using "DHCP with manual address", and type in a normal "local" IP address such as "10.0.1.7". On the other computer, go to the Network Preferences and make sure "Built-in FireWire" is enabled, and make sure the connection is using "DHCP" for connecting.
I couldn't seem to get the internet sharing to work without running a proxy server, so I also run tinyproxy on my iMac, and in the PowerBook's Network Preferences I make sure it's set up to use the proxy.
From what I've read, I'm inferring that in order for the two Macs to talk to each other (using Bonjour/Rendezvous), they both need to be using DHCP. This may or may not be true, but it's my conclusion for today. On top of which, I need a fixed IP address for my main computer since otherwise the other computer won't be able to find its proxy host.