Google recently dimmed its lights observing the Earth Hour. Whether using a black background conserves energy or not is debatable and depends on the type of monitor that is used. One attempt at saving energy by dimming the background on one of the most popular pages online is Blackle. Blackle was developed by Heap Media, a company based in Sydney, Australia and founded in 2005. Heap Media "is committed to developing and growing leading online services with global reach."
Search Google in Black
www.blackle.com