I've been using Ubuntu on my mac for a while and everything pretty much works without any cofiguration on Intrepid, but there were a couple of annoying problems that I had to sort out.
# and € keys on UK macintosh keyboard layout
Hash and euro didn't work which can be a pain, so I've mapped them to the key left of the Z key (marked ~`)
You probably should create a new keyboard layout but I just added the new key mapping to the existing gb symbols file.
Open it with:
sudo gedit /usr/share/X11/xkb/symbols/gb
Scroll down to the United Kingdom - Macintosh bit at the bottom and replace the two lines underneath with:
key <AE02> { [ 2, at ] };
key <AE03> { [ 3, sterling ] };
key <LSGT> { [ numbersign, EuroSign ] };
and log out for it to take effect.
Touchpad scrolling and double tap right-click
To get the touchpad working properly on Intrepid I had to configure HAL.