I actually had the same situation a few hours ago. Brother and everything!
Whatcha wanna do is go into preferences, make sure iChat rendezvous is turned on on both machines (in Tiger, it's called Bonjour). Turn it on, open iChat. Go to iChat preferences and enable it. In tiger, it's under "accounts"
I'm not sure of what it is under 10.3, but it should be easy to find--iChat preferences isn't that big