There has been some criticism of it claiming to have reduced emissions by so much depending on how many people sign up. My understanding of this is that it takes a few hours to switch a powerstation down and then similar time to start up again and if they do not know exactly how much less will be used they can't really cut back the power production.
I personally see this event as a part of a growing recognition by people and communities that things need to change. It's exciting that people are getting involved. However, if all people do is turn their lights off for one hour a year nothing will change. Instead use this time as a household to brainstorm ways of reducing your emissions, your waste, and your energy usage. Look at ways you can get involved in your community, in lobbying govt for change. Be creative. If this doesn't build momentum for change then why bother.
I personally will be enjoying earth hour over some candles and a bottle of wine with my friend Sue in her strawbale house talking about the national permaculture convergance she has just been to. Better finish work quickly so I can get there sooner. Talk later. Hannah