When you press-and-hold a key when editing in OS X Lion, instead of the usual key repeat behavior in previous version of OS X, you get the accented character popup for your selection of alternate characters.
This behavior might not be desirable for most apps. To change it back to Key Repeat behavior, enter the following command in Terminal.app.
defaults write -g ApplePressAndHoldEnabled -bool false
This will change the behavior system wide. To change it back to the default accented character popup, enter the follow in Terminal.app:
defaults write -g ApplePressAndHoldEnabled -bool true