What a lot of people don't seem to understand is that electric cars only offset the carbon footprint. Instead of producing CO and NOx from the vehicle, they'll just be produced by powerplants due to the increased demand for electricity. There's no such thing as a free lunch and electricity has to come from somewhere. And like danwjmu said, what happens to the batteries when they die? They won't live forever...
Believe it or not, some locations have advanced past the stage of burning shit for electricity. TVA draws quite a bit of power from nuke plants and hydro dams. Our city is powered by nuke and water...
Also, people would typically be charging their cars at night. Power plants can't exactly be "switched off" at night when power consumption is lower, so a lot of power is generated that isn't used.
I'm not implying that electric cars are great and will solve the world's problems, but they're a step closer than continuing to use fuel-powered cars, and some of the arguments against them are not that strong.