Hit ^F7, or System Preferences > Keyboard & Mouse > Keyboard Shortcuts > Full Keyboard Access > All controls You know how the default action in a dialog is a s

Hit ^F7, or System Preferences > Keyboard & Mouse > Keyboard Shortcuts > Full Keyboard Access > All controls You know how the default action in a dialog is a solid blue aqua button, and hitting enter presses it? When this preference is turned on, there's an additional blue tab-selection halo that fires its button when you hit spacebar, and it always defaults to the cancel/don't button.