To sort of spoil the fun, they don't actually turn off any coal/oil/gas/what-ever-other-fossil-fuel-they-are-burning power plants for the one hour that we all turn off our lights (coal power plants, for example, can take days to warm up to operating temperature). Therefore no CO2 is actually saved... But its the thought that counts...
Then when we have to consider the toxic chemicals in the batteries you use in your torch during Earth Hour...