UPDATE 30/01/2011: I've had to correct the maths a little. Fortunately, the numbers that come out are almost identical.
Ever since the invention of flat panel monitors and screens, it didn't matter what colour your pixels were, you'd still consume the same amount of power. That's because these screens have a big flat light at the back that's always on. If a pixel wants to show you white it lets all the light through. If it wants to show you black it blocks all the light. Either way, the light behind is still on and still consuming the same power.
On a mobile phone that means the colour of your wallpaper doesn't affect battery life. But all that changed with the introduction of OLED screens (or more commonly AMOLED and Super AMOLED) and a lot of new phones have these. If your phone has an OLED screen keep reading, you might be able to improve the life of you battery.