When you press and hold a key in Mac OS X Lion or Mountain Lion, it is no longer repeated. Instead, an auto-correction pop-up would appear, allowing you to select accented characters. This default write command will re-enable the repeat key feature.
defaults write -g ApplePressAndHoldEnabled -bool false
Restart your Mac or logout/in to make the changes appear.
Return to normal behavior with:
defaults write -g ApplePressAndHoldEnabled -bool true