To add text to the login window in Lion, there's no need anymore to use a system utility or to manually edit a plist file, as it was the case in earlier versions of Mac OS X. This can be useful to display an "If found" message.
Go to the Security & Privacy preference pane of System Preferences. Under the General tab, check the second box, "Show a message when the screen is locked," and then edit the message using the "Set Lock Message..." button.
This message displays near the bottom of the Login window or the lock screen. You can make multiple paragraphs by pressing Option-Return to go to a new line.
[kirkmc adds: This was indeed covered several times in the past, and required futzing around with .plist files. There were hints in 2002, 2005 and 2011. This method is much simpler, and deserves a mention.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120530210725545