I read an article a year or so ago suggesting a possible link between air conditioning and obesity. It talked about how, in the days before central air and forced-air heat, the body's normal cycle was to bulk up when the weather started to get chilly in the fall (to provide an extra layer of fat insulation) and then to shed that extra weight when the weather broke in the spring. But since the advent of controlled heating and cooling, we're fooling our bodies into keeping the weight and then adding on to it. By not experiencing the weather fluctuations, our bodies do not have to work (and thus burn more calories) to warm up or cool down. And it starts to create a cycle... the heavier we are, the hotter we get, so we crank the air down another degree.
I think this makes a lot of sense. I have always been one for turning the air on as soon as the weather hits 80 because I hate to sweat. I'll definitely be reconsidering that this summer.