That's the question that Michelin executive Patrick Ozoux wants the EU to answer. According to a story in Automotive News Europe, Ozoux said that, "When carmakers fit such tires as original equipment, they should get a bonus on their CO2 rating." The EU will likely be assigning a CO2 rating to new cars in the coming years.
I suppose the important consideration for the regulators is how long does the CO2 reduction last. Is it for the life of the car? Probably not, since the tires will likely be replaced at some point. Then again, I'm sure the CO2 ratings would naturally be based on the car as sold, right? In that case, it only makes sense to accurately assign a rating that includes the reduction from the low rolling resistance tires.