New in OS X Lion is the ability to display a message when the screen is locked. This is useful to inform your co-workers not to meddle with your Mac in your absence. Go to System Preferences > Security & Privacy. Under the General tab, check the option “Show a message when the screen is locked” and key in your note below. If you’re not able to check the option or key in the note, click on the lock icon at bottom left corner to unlock for editing.