How to Turn Off Automatic Brightness
I assume you are using OS X 10.6. I had the same issue on my Macbook Pro. While i didn't experience "flickering", I did notice changes in brightness while typing an email. At first I thought I just might be going crazy. But it seems that just moving toward and away from the screen through changes in my posture was enough to trigger a change in ambient light as far as the computer was concerned. Another compounding factor was that I had turned off the main overhead light in the room above the computer table. A few feet behind and above me, I had two skylights turned on so small movements on my part really affected the amount of light getting to the computer's sensor, wherever that might be.
I was able to find the setting to change it to manual brightness. Here's how to do it:
Go to System Preferences | Displays | Displays tab
just under the Brightness slider is a checkbox for "Automatically adjust brightness as ambient light changes." Uncheck that, and the problem should stop.
I must admit, it's a neat feature, but I like to adjust brightness settings manually, especially if I'm working on some graphics.