You need to edit the com.apple.loginwindow.plist file.
Simply enter the following in terminal (replacing the asterisks with your message):
$ sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText "whatever text you want"
Alternatively, there are a few GUI-based tools that can help you achieve the same thing:
Try TinkerTool System or Visage Login (both available from VersionTracker)