The guys at pyroCPU play around with ATi's PowerPlay application, which is now included in their Catalyst drivers. What is PowerPlay, exactly?

"For the uninitiated, ATI's PowerPlay allows the user to specify various 'power-states', or rather various frequencies and voltages at which the card can operate. The purpose for these various performance levels is ultimately to allow mobile users to save on battery life through running at reduced speeds when not performing strenuous 3D tasks."