So there's this bit of stupid, arguing that (at least in Great Britain) electric cars do not have smaller carbon footprints than gas- (er, petrol-) powered ones. The author does the following mental gymnastics:
First he takes the energy consumption of a compact petrol-burning car (55/43 kWh per 100km city/highway). Then he takes estimates of electric vehicle performance from two other studies, which give them at ~16kWh/100km and ~20kWh/100m. Finally he claims that since fossil fuel power production is only about 36% efficient, those numbers are really 48 and 60 kWh/100km - worse than the compact car!
This is purported to show that there is no real benefit at all to driving the electric vehicle. Good thing there are holes in his analysis large enough to drive a Chevy Volt through.
Unrelated Sources of Data
The numbers are not drawn from the same sources; in the studies that the electric vehicle numbers come from, the estimated power consumption of gas vehicles is much higher: 60-80 kWh/100km. This suggests that the formulas used are different, which means the numbers can't really be compared. Using only the numbers from the quoted studies, even at 36% production efficiency, the electrics would still be significantly more efficient.
Cost of Production and Transportation
The numbers for the petrol car don't take into account the cost of refining and transporting fuel. These are both carbon-intensive processes. The production and distribution of fossil fuel energy for traditional car engines is only 70-80% efficient when these costs are taken into account, giving the electrics a further edge.
Fuel Sources for Electric Power Generation
The numbers for the electric car don't take into account the types of energy being produced. Burning coal generates 25% more CO2 than burning petroleum for the same energy; burning natural gas generates 25% less carbon. Nuclear and renewables generate none (or nearly none).
What this boils down to is that in terms of carbon footprint, if your country generates less than 80% of its power from coal, the amount of carbon you're generating per kWh spent for an electric is less than what you would generate with a gas or diesel vehicle. Great Britain only generates a third of its electric power from coal and another third from natural gas, which means that each kWh of electric is only putting about two thirds of the CO2 into the atmosphere that an equivalent kWh of petroleum fuel would. The United States is worse, generating half its power from coal and another 17% from natural gas, but that's still only three-fourths of the carbon of burning gasoline or diesel to produce the same energy.
Opportunities for Future Development
Even if the carbon footprint of the vehicles were identical now, future trends are only in favor of the electrics. We're moving away from coal energy production for a number of reasons, and gasoline is only getting more expensive and energy-intensive to extract and refine as easily-reached, high-quality sources of petroleum are exhausted. Our production and transmission technologies for electric power are improving, as are our battery technologies, which will yield lighter, more efficient electric vehicles. In contrast, internal combustion engines are near the limit of efficiency - we're not going to see a whole lot of mileage improvement out of traditional gas and diesel vehicles over the next few decades.
Add to that the decreasing cost of renewables - especially solar power - and you'll see a very different picture in ten or twenty years. We can expect to see a world in which electric vehicles have carbon footprints that are half, a quarter, even as little as ten percent of traditional cars. So even if you live in a place like I do where most of the power comes from dirty coal, and right now an electric car is going to produce nearly as much carbon to operate as a gas-powered one, it's likely that over the lifetime of the vehicle those numbers will change and you will still have a net positive environmental impact.
To sum up: there may be a lot of good reasons not to buy an electric car right now, but carbon footprint isn't one of them.