Some say that electric cars will produce as much or more CO2 as gas cars, but I doubt this is true.
It is true that much of the electricity used to charge electric cars will come from burning fossil fuels, but not all. Here is how we made electricity in 2009 in the US:
It will depend on what part of the country you live. The worst scenario might be it you lived were they burn mostly coal, such as my town, Tucson, Arizona. Here, my electric vehicle batteries would be charged from a coal burning power plant. But there are offsetting factors. First, the efficiency of a car engine is about 25%, were a coal power plant is about 40%. But storing energy in batteries means a storage loss of about 20% thus requiring more coal to be burned. But I think the most dominate effect will be the naturally higher overall efficiency of transportation for electric vehicles. Electric cars should be lighter, smaller, slower, and will probably all have regenerative braking so the effective "MPG" might be quite high. Also, because of the speed and range limits of electric vehicles, I think most drivers will drive smarter by combining trips and simply driving less.
If the electric vehicle drove the same speed and range as a gas auto, the CO2 output would be more level, but I don't think this will be the case.
So at the end of the day, I think electric vehicles in regions were power is from nuclear, hydro, or renewables will result in much less CO2 output, and even in areas were coal is burned, I think total CO2 output will be reduced as well.