I think most of you may have read the forum in the Today paper this morning. I checked out several websites and really, it depends on what kind of candles you use, how many of them you burn, and where you get your electricity from.
According to Christian Science Monitor, most candles are made of paraffin, a heavy hydrocarbon derived from crude oil. Burning a paraffin candle for one hour will release about 10 grams of carbon dioxide. Instead, you should be burning beeswax and soy candles which are mostly carbon-neutral because any carbon they release by burning was only recently absorbed by plants from the atmosphere.
I also read this which says that burning paraffin candles actually increases CO2 emissions. It says that "for every candle that is burned to replace electric lighting during Earth Hour, greenhouse gas emissions over the course of the one hour are increased by 9.6 g of carbon dioxide."
Anyway, my verdict is at the next EarthHour, don't turn to candles. They produce heat, indoor air pollution (if you're indoors), a fire hazard and many times less efficient than a light bulb!