(Problem restated now that I know a bit more)

One of my monitors is way darker than the other one. The mouse cursor is bright like it normally should be, but everything else is dull and the whites appear gray. It wasn't always like this. What gives?

Running Windows 7 and a Geforce GTX 460 video card.

Going into my Nvidia control panel, I have "Let other applications change video settings blah blah blah" selected. If I select "Use NVIDIA settings" it goes away and things are nice and bright. So this leads me to believe some background application or something that's not running anymore was darkening it. What is the significance of changing this option?

This is just an idea, but I've found that if you alt tab while running certain games, your OS will take on the gamma properties of said game. I'm not sure if that's related to your issue, but I've had it happen before so you never know. I don't really think there's a fix for it, other than restarting the computer.

This has happened to me before. Resetting something in my video card's control panel fixed it, but I never figured out what caused it in the first place.

ditto on what the others have said, it's happened to me once or twice and was caused by windows taking on the gamma of a certain game.

Or starting any other game, actually.

This usually happened to me when I was playing Just Cause 2. Sometimes when I'd quit the game, everything would remain rather dim. Starting any other game brought things back to normal as soon as the game loaded, though.

I glanced at the forum briefly and thought for a second that someone had started a parody thread entitled "Does my underwear seem darker than usual?"