Three comments on lower fuel efficiency during winter:

(1) the cold weather brings lower tire pressure. I found my tires needed air and got a 3 mpg boost when I filled them. Remember to check tire pressure when they're cold. Tires can lose 1 psi per month and 1 psi per 10 degree temperature drop (http://eartheasy.com/live_fuel_efficient_driving.htm)

(2) the cold weather also makes the car work harder. Energy is used to warm up the engine, its lubricants, and our hybrid battery. We also use headlights, defrosters, heaters, and maybe windshield wipers more often. On top of that, the cold outdoor air puts more air resistance to the car.

(3) the winter gasoline blend is phased in during the fall. Winter blend is made to provide higher vapor pressure to keep the engine firing in cold wether (i.e., more butane content). It's nice to have a car that runs, but the bad side is a ~2% drop in energy content on a volume basis (http://www.chevron.com/products/prod.../fuel_economy/)

Don't blame the hybrid!