In Lion you can generate a log-in ￼screen message, such as “Property of (your name) – (your phone number)”.
Or, “Touch this computer and you are dead”. Or, “This computer is cursed”. Whatever strikes your fancy.
Just select Apple Menu > System Preferences > Security and Privacy > General Tab. If the preference is locked, as it should be, unlock it to make changes. Enter your message as demonstrated in the image.