Yes, there are zero emissions from the tailpipe, but the electricity has to come from somewhere. Hydro, solar, wind are all very small percentages of the total electricity generating solutions. The majority of electricity comes from 'dirty' sources, such as coal, oil and nuclear. All thats happening is the carbon output of transportation is being shifted from the consumer to the manufacturer.
And even if we DO get a much larger percentage of clean energy sources, we still have to consider the massive costs that will be needed to upgrade the aging energy grid to be able to support the huge draw that electric cars will demand. Even a few million electric cars in the US will put a huge strain on the grid.
Finally, how good are the batteries for the enviornment? Can they be recycled cleanly? And how often do they need to be replaced? After a few months of steady use?