Why does decreasing the brightness cause the brightness to increase occasionally on the MacBook Air? I have a MacBook Air Late 2010. I notice that sometimes when I press the decrease brightness button, it causes the brightness to increase and only after subsequent presses it starts to actually reduce the brightness (from this now higher brightness setting).

I was just wondering why this happens, and if it's related at all to an ambient light sensor similar to the one that the iPhone has.
The Air does have an ambient light sensor. Try disabling it in the Displays System Preferences pane and see if it fixes the problem.