It's lights off for Google in the UK!
On Saturday, March 29, 2008, Earth Hour invites people around the world to turn off their lights for one hour – from 8:00pm to 9:00pm in their local time zone. On this day, cities around the world, including Copenhagen, Chicago, Melbourne, Dubai, and Tel Aviv, will hold events to acknowledge their commitment to energy conservation.
So now that Google's gone black, will it ever go back? Apparently.
As to why we don't do this permanently - it saves no energy; modern displays use the same amount of power regardless of what they display.