According to the IESO website, 560 MWh of electricity use was avoided as a result of Earth Hour and I'm going to tentatively accept that figure for the sake of argument. The real question I want answered is how much carbon dioxide was really avoided with this exercise.
The way not to answer this question is by taking a chart of total capacities and dividing the 'savings' out among all the generation sources weighting them by capacity. This is because for one thing, nuclear power was not affected by Earth Hour.
Using data from the IESO generator reports we can get a pretty good idea. For simplicity's sake, I'm going to combine gas and coal because the amount of coal used was not very significant. And I'm going to combine the non-carbon sources, nuclear and hydroelectric as well.
Figure 1: Electricity Generation by Source for March 27th (Earth Day) 
As you can see, there was a slight dip where typically there is an increase, by comparing this to the electricity production for the previous week this becomes even more clear.
Figure 2: Electricity Generation by Source for March 20th 
Comparing these two we can make no solid conclusions, and by comparing a hundred graphs we probably could make nothing conclusive either because of the large number of variables (weather, overall production, etc...) that need to be taken into account. But yes, we reduced our electricity production by some amount.
However, electricity from coal and gas sources is consistent with the previous week in terms of overall production and the changes hour-to-hour. If anything, it looks like hydroelectricity production took a dip at 8pm. So on that basis, I'm willing to say that I believe no statistically significant amount carbon dioxide was avoided as a result of Earth Hour.
So to all you who dutifully turned out your lights on March 27th, congratulations, you did absolutely no good whatsoever in terms of carbon dioxide output.
 All data is publicly available on the IESO website.