When you're playing a full screen game, the mouse and keyboard are devoted to the game...no way around it.
When you're watching a DVD or something, and writing a word document for instance, you can switch back and forth.
But there's NO way to plus full screen 3D games and "move over" to the other screen to type...unless you pause the game and ESC it or something.
While you're playing the game , though, you'll be able to keep your eye on the second screen...and if it's AIM or something, you'll see your buddy typing to you..you just won't be able to reply