As was said, there is more gas-wasting warmup time in the colder months. In the summer, when you start your car on an 80 degree day, the coolant comes up to temperature quickly, the 02 sensors heat up right away, and the ECU goes into closed loop fuel control within a minute or two of driving. In the winter, you start the car, let it idle for a while or whatever you do, and it takes much longer for the ECU to reach closed loop. Until it does, it runs in the much less efficient open loop mode where the amount of fuel injected is estimated based on a default (and overly rich, just to be safe) fuel map, rather than using the 02 sensors to determine the air-fuel ratio.
I would say that most of the decrease in fuel mileage can be attributed to this. Additionally, people tend to make more short trips in the winter, causing a decrease in economy. There's also the theory that fuel doesn't vaporize well when injected in cold weather- this is true, but in my opinion the first few seconds of engine operation heat things up enough to negate this effect. It was certainly true with carburated engines, but fuel injection really isn't affected much except maybe in extremely cold climates not really seen in the lower 48 states.
Cold makes us just plain lazy too- I noticed my tires looked a little low like two weeks ago (Usually I check them every week or so) but it's been cold and rainy here in NJ and I just never found the motivation to put more air in them. It's little things like that, they add up. Winter sucks, and it hasn't even (technically) started yet!
