I have one of those 24" iMacs that hums loudly whenever the display is any dimmer than max. Luckily I like maximum brightness, but I've always had to set Energy Saver to never turn off my display because the dimming creates a loud hum. I have, however, found the solution. Go to Terminal and type this command (and enter your password when prompted):
sudo pmset -a halfdim 0
The halfdim mode is now disabled, and your display(s) will simply turn off at the interval designated in Energy Saver without ever dimming. Now I can stop leaving my display on permanently and get a bit more life from it.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070217052629138