Re: @ bazza
Last estimate I saw was that the current US grid could support 70% of all transportation miles without needing a single extra power station.
As an example, I have a Chevrolet Volt. It charges at 3.3kW. I charge my Chevrolet Volt off peak, starting at midnight during the week and any time during the weekend. If my charging adds to the grid peak load it's only the standby Wattage of my EVSE. I'd charge off peak anyway, but it's cheaper for me to do so. Sensible pricing will naturally lead the market to charge away from peak demand.
The emissions from the required electricity are not a fundamental issue because it's substituting for petroleum-based fuels. Not only are there direct benefits from the substitution, but if plug-in vehicles are successful it will imply the existence of two things: cheap batteries and a massive, controllable demand sink. Both of those would provide substantial benefit to the electricity grid through raising generation efficiency and allowing easier integration of renewables.