Me deparei com um protetor de tela chamado Power Dimmer :
Power Dimmer is a Windows screen saver application that, once activated, will gradually dim your screen, fading it from full brightness to a final brightness level that you set. While the screen is dimmed, you can still see all of your windows, as well as any on-screen activity. Move the mouse or press a key and your screen will be restored to full brightness.
Embora diga apenas para o Windows 200 / XP, acabei de testar na área de trabalho do Windows 7 de 64 bits e funciona bem para mim.