A team at Carnegie Mellon University modeled the net emissions of plug-in hybrid electric vehicles (PHEV) under a slew of different scenarios and discovered that, although the widespread use of PHEVs will likely result in a reduction of CO2 and NOx emissions, the overall environmental impact of PHEV's is highly dependent on several variables. The team concluded that charging strategy, battery pack capacity, driving patterns and the type of liquid-fuel engine used in a PHEV will impact net emissions and, in some cases, could even negate the environmental benefits typically associated with driving a plug-in hybrid.
In its research paper published in Environmental Science & Technology, the Carnegie team estimated the effects of ten percent of the light-duty vehicle fleet becoming PHEVs. The short version is that if coal power plants are used to power PHEVs, we'll get, "lower benefits unless coal units are fitted with CCS [carbon capture and storage] or replaced with lower CO2 generation. NOx is reduced in both RTOs [regional transmission organizations], but SO2 increases."
Whether or not PHEVs will actually contribute to a reduction in net emissions depends upon the average fuel efficiency of the conventional (gasoline- and diesel-engined) vehicle fleet. That is, if CAFE standards soar, then use of PHEVs may increase net emissions of both CO2 and NOx.