(I'm going to assume you're not running OS X Server here on that MacBook Pro, and are "just" running OS X 10.6 client here.) Did you possibly trigger one of the accessibility options available within System Preferences > Universal Access? If so, take a look around under the "seeing" options there. (It's also possible to adjust the screen brightness via the keyboard, but I'm guessiing you've looked at that?)
If that's not it, have you tried a reboot of your MacBook Pro, and see if that might clear the highlighting.
And if the reboot doesn't clear this, then some more details on what you're seeing here and maybe a screen shot showing what part of the screen is highlighted, and whether any particular application(s) are in use here that might trigger this?