The Washington Post publishes this somewhat useful but very pretty map today:
Mapping how the United States generates its electricity - Washington Post
For us, at least, it updates some figures on the energy mix in different areas of the country. Plug in an electric in California and you are likely powering it largely with Natural Gas. Move to Maryland with the same car and it the energy going into your car might make it effectively a coal powered car. [EDIT: In Ky or WV you would really likely be driving an almost 100% coal powered Tesla. Yikes.]
I suppose solar generation development in the future could displace a lot of these more polluting sources.
At the Orange County auto show here in Cali this weekend I heard on the radio that there is a concept car, from maybe Mitsubishi, that is a PHEV that uses hydrogen fuel cells. That would be kinda cool.