Sunil Somalwar, a physics professor at Rutgers University, presents the following argument at the Better World Club site:

Let us conservatively say that a Prius goes 40 miles on a gallon of gasoline. After taking into account the 20 lbs CO2 released by burning a gallon of gasoline, 40mpg amounts to two miles per pound of CO2 emission. On the other hand, a plug-in electric car may not emit any CO2 from the tailpipe, but when I draw a kilowatt-hour from the electric grid here in New Jersey to charge the car batteries, a coal plant in some other state belches out 2.5 lbs of CO2. According to Toyota, the plug-in version of the Prius will run about 2.5 miles on that kilowatt-hour of electricity, which means that I get only one mile per pound of CO2 emission. When I plug it in, my 40-50 mpg Prius becomes half as efficient and turns into a 20 mpg SUV. (The story with GM's upcoming Volt plug-in car is no different.)

Here's the rest of the story (it's pretty short). Has the good professor overlooked anything?