Guys and Gals, the reality is there isn't much less traffic online in the summer. I've lived in Florida and have friends in other hot weather climates and the heat pushes people in those areas inside. For them, it's like winter is for us and they spend more time online in the summer than in their "winters". Even in northern climates as well these days, when it gets hot and humid many more people are retreating inside to the comforts of AC.
I used to buy into the "it's slow in the summer" belief as well, until I did some reseach and saw that there were plenty of sellers doing quite well in the summer (and all year round in fact). Obviously these individuals were thinking outside of the box. My conclusion was that it comes down to whether your business is "season proof" or not.