Earth Hour is a somewhat polarising event, with some hailing it as an important stepping stone towards avoiding global catastrophe, while others openly mock it.The first thing to note on Earth Hour is that it is obviously intended to be a symbolic gesture. Anyone who says that turning your lights off for an hour won't avert global warming is confused - no one is suggesting that it will, such a thing is obviously ludicrous. In fact, (the admittedly tendentious) Not PC blog shows us that electricity consumption spiked just prior to Earth Hour, showing that total electricity consumption was probably relatively constant over the whole day. Obviously if Earth Hour has any value, it is a symbolic one.
My worry however, is that supporters of Earth Hour make the reverse error to its opponents - assuming that their actions do more than they actually do. I worry that people will not intuitively realise that they are being symbolic and actually consider their actions part of the solution, and thus revise downward what else they would do. Earth Hour is a poor substitute for serious action.
For my part I think that any 'solution' to the problem of climate change can't on simply convincing people to reduce their own externalities. You have to implement either a cap and trade scheme, or a carbon tax (which I prefer) and make people bear the cost of their transactions. This is basic economics, but it is taking a devastatingly long time to penetrate the web of vested interests that is the public policy machine. I fear that gimmicks like Earth Hour only prolong the process.