The brightness of the whole screen goes down considerably when I enlarge windows with a dark background (Gnome-terminal and TTYs for example, as well as Games, especially when in fullscreen). It's up to a point sometimes that the text on TTYs is only barely visible. The brightness goes up again if I put other windows (like a regular Firefox-window) in front or when I make the "problematic" windows smaller. It also happens if a Browsser-window shows lots of dark/black background, but it seems that the effect is more severe the more of that darkness is shown.
I use Ubuntu 12.04 and nVidia-drivers 302.17 (problem occured for many months, with 11.10 and older nVidia-drivers as well).