From their website:

It started with a question: How can we inspire people to take action on climate change?

The answer: Ask the people of Sydney to turn off their lights for one hour.

On 31 March 2007, 2.2 million people and 2100 Sydney businesses turned off their lights for one hour – Earth Hour. This massive collective effort reduced Sydney’s energy consumption by 10.2% for one hour, which is the equivalent effect of taking 48,000 cars off the road for a year.

With Sydney icons like the Harbour Bridge and Opera House turning their lights off, and unique events such as weddings by candlelight, the world took notice. Inspired by the collective effort of millions of Sydneysiders, many major global cities are joining Earth Hour in 2008, turning a symbolic event into a global movement.

I am asking you to participate in this global statement. On March 29th, 2008 at 8 pm in your local time zone, turn off all the lights and electrical equipment for one hour. Encourage your friends and neighbors to do the same and see what a difference the flick of a switch and one hour can make.

To declare your participation, go to EarthHour.org.

Do you?