I have periodically noticed that my display has been a bit dim, and have brought it up to maximum brightness... only later to find that my display was about halfway on brightness. I can't rule out a stray keystroke, but it seems to be several notches down a lot more often than I would accidentally hit F1. As far as I can tell this happens when the computer remains on and I remain logged on.
Especially when operating on battery power, does Mountain Lion dim the display automatically? Is there a way to turn off or circumvent this?