At least till Ubuntu 9.10 (Karmic Koala) With Ubuntu 10.04 it is still not possible to dim the display with any ready to use solutions, like the gnome-applet in the panel. Crazy enough there was one very time, the applet worked and also the display dimmed a bit when idle, but never again.
Felipe Alfaro Solana in a nice effort coded a little program, with which you can configure the iMacs display brightness. The source code of this program can be found on his page. However in a thread on ubuntuforums.org dealing with the brightness issue, you can also find this link with a ready to use program.
The usage of the program is not that geeky, but also not as nice as an applet. Supposing that you allready made the file executable and it is called backlight, you use it like this:
If you copied the program in your home directory, just open up a console and type
sudo ./backlight 10
Your display now should only be 2/3 of the brightness it was before. You can choose between 1 (dimmest) and 15 (brightest).
Notice that after a restart the brightness is reset to brightest again, so if you permanently want to have your display at 10, you should consider adding the above line (without the sudo) in /etc/rc.local. Doing this will set the display to brightness 10 on every startup.
Starting with Ubuntu 11.10 you don't need to use this complicated method anymore, since brightness configuration is now supported out of the box. Just go to the system settings, choose "Screen" (not "Displays") and change the brightness by moving the slider as you like.