Not quite the entire Earth, but millions of people around the globe observed March 29th's "Earth Hour", a global effort to show how we can have an impact on the climate. The premise was to turn off your lights between 8 and 9 PM during your local time, which in turn would conserve energy in a big, unified way. Many cities participated with hosted events to observe the hour. Not only homes, but restaurants, local businesses and even landmarks turned their lights out as part of the event. According to news reports, even the Queen of Denmark shut lights out to be part of the effort.
Earth Hour was started over 10 years ago by the WWF (not the pro wrestling league, but World Wildlife Fund). According to their site's info page on the effort:
Created by WWF in Sydney, Australia in 2007, Earth Hour has grown from a single event into a global movement. In 2008, millions of people, businesses, governments and civic organizations in nearly 200 cities around the globe will turn out for Earth Hour. More than 35 US cities will participate, including the US flagships--Atlanta, Chicago, Phoenix and San Francisco.
More at WWF's Earth Hour page
Something like this should really be done on a weekly basis in homes just to cut down their energy bills and save money. WWF encouraged people to replace their standard light bulbs with energy-efficient fluorescent bulbs during the blackout hour. It's amazing just how much power and electricity we use on a daily basis, but a little bit of conservation can go a long way. Hopefully this becomes an annual tradition with more and more homes and people participating.