The Login Window in OS X is the screen you see while you’re typing in a username and password. There are a number of customizations used in some environments to make the system easier for users to use, or to make it more specific to a given user environment. One such is customizing the Login Window’s background, which can be done by replacing this file with one that you would like to use:
You can also configure a message to be shown to users. This message, often referred to as an Acceptable Use Policy, can be used as a policy banner that users must accept in order to log into a computer. To set a policy banner, create a file called PolicyBanner.txt, PolicyBanner.rtf, or PolicyBanner.rtfd with the information you want displayed for end users. Save this file to /Library/Security. Then, the contents of the file will be used as a login banner users will be required to click on the Accept button in order to login.
You can also use Profile Manager and Managed Preferences to manage the items from the System Preferences pane and set a message at the LoginWindow as well. These are available under the Login Window section of Profile Manager.
Update: Those crazy kids at AFP548 have posted a video on YouTube with additional info on Profile Manager. That video can be found here.