It's actually worse.
There have been studies in the UK showing when Earth Hour happens, there is a drop in electricity demand leading to a slowdown of power generators. But when Earth Hour is over, the generators speed up to meet the surge in energy demand. During this time the co2 produced more than cancel out the co2 savings during Earth Hour. So Earth Hour actually causes more co2 to be released.
Also, the candles that people love so much during Earth Hour? Each of those release more co2 than the use of a single incandescent light bulb lit for 1 hour.
So it's not like donating 1c to tsunami victims. It's more like robbing them then patting ourselves on the back. Congratulating ourselves for indulging in such a meaningful, noble activity.