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:
/System/Library/PrivateFrameworks/LoginUIKit.framework/Versions/A/Frameworks/LoginUICore.framework/Versions/A/Resources/appleLinen.png
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.
/Library/Security/PolicyBanner.txt
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.
Update2: For
