This software displays two versions of the world map, a day, and a night version, and fades between them to show which portions of the world are in daylight, and which aren't. In other words, this is a dynamic wallpaper that updates based on where the sun is relative to the surface of the earth.

herethere.net/dt

On my dualview setup the wallpaper expands to the TV as well, but I don't mind. I use a manual X offset, fullscreen mode and some overlays.