My Admin user has a basic, flat gray background image, while my other Users have more colorful pictures and the like. This helps me know in an instant when I'm in Admin mode (and should be more careful), and when I'm not. This might also be useful for great hints like Cue the next song while DJing with iTunes, to help identify when you're logged in as the primary or secondary User.
[robg adds: I do this with the users I have created for use with fast user switching. It's especially important as I've disabled the FUS menu, so the desktop picture is really my only visual cue as to who I'm currently logged in as.]

