Gah, ME TOO!!
grr. What IS this problem.
Suddenly I can't change the brightness, neither with the F1 and F2 keys, nor with the slide bar in bootcamp control panel.
Granted I have modified quite a few things like installing trackpad++ and power plan assistant, which do hook into the bootcamp control panel for their operation. But I still don't think they would be the cause.
I have MBA 13" Oct 2010 model.