screen dims

I have a 20" duo-core imac (white) at work and a 20" duo-core (aluminum) at home and have the same issue on both machines where the screen will dim if the computer is inactive for a brief period. Its not that the computer is going to sleep with the screen going black but more that the brightness level changes suddenly from normal to the lowest setting. If I move the mouse or use the keyboard the brightness level pops back up. The screen dimming almost seems like an energy saving feature but I cannot find any reference to it in the help section and there is no setting or control that I can find in system preferences. A friend of mine with a new MacBook reports the same problem.

Thanks