From iPhone Development Wiki

SBBrightnessController is a singleton class that is used to manipulate the brightness level of the screen. You can access the singleton instance with the nonstandard method as follows:

+ (id)sharedBrightnessController

Brightness Level Manipulation

The following method can be used to manipulate the brightness level, where allowed values are in the range from 0.0 to 1.0. An optional HUD can also be displayed.

-(void)_setBrightnessLevel:(float)level showHUD:(BOOL)hud;