That's because a plug-in's lower tailpipe emissions may be offset by smokestack emissions from the utility generating plants supplying electricity to recharge the big batteries that allow plug-ins to run up to 40 miles without kicking on their gasoline engines. Plug-ins, called PHEVs, are partly powered, in effect, by the fuel used to generate the electricity.
By James R. Healey, USA TODAY
Fujll Story: http://www.usatoday.com/money/autos/environment/2008-02-25-plug-in-hybrids-pollution_N.htm