This post explains how to connect to a Leopard Remote Desktop session from a Ubuntu client machine using VNC. It is assumed that Remote Desktop is enabled on their Mac (you should also read this regarding a password on your Leopard Remote Desktop server).
The Leopard Remote Desktop speaks a flavor of vnc, albeit one which common vnc clients seem to have difficulty with. I tried both Ubuntu's default vnc client in addition to a realVNC client, both without success. However I did find that the tightvncviewer successfully connects.
To get xtightvncviewer, install this package:
sudo apt-get install xtightvncviewer
When you want to initiate a Leopard Remote Desktop session, open a terminal and start up xtightvncviewer:
You should be prompted with your password (you did set a password on your Leopard Remote Desktop, correct?), and following that you should then see the desktop of your Mac in the vnc window.