I have a 12" iBook and every so often my screen gets darker than usual (not sleep mode), and it will stay like that for a long time, sometimes hours, and then go back to normal. I move it to the brightest setting when this happens and it still is very dark and difficult to see. Does anyone know why this is happening? I hope it doesn't mean my screen is going. This has been happening for a while now, at least a few months.