As simple as it gets - a basic Gimp gradient fill in greyscale. Works quite nicely as both wallpaper for Compiz Sphere, and Skydome image (set to animated Skydome - it will still distort the background).

Cube (Sphere) Caps should be set to colour #6f7269 at the top and #8c888b at the bottom to match the wallpaper and get something like my screenshot.

Hope someone finds this useful!