Reader Ace writes in with a nifty customsation that will set your Mac apart from the rest. Find a desktop wallpaper image (Social Wallpapering's a good bet), save it to your desktop as
background.jpg, and in the Terminal type:
You'll have to enter your password to complete the operation, and when you do, next time you log off, your logon screen will have your new custom background. Slick. Thanks, Ace!