A June 15, 2007 article in The Boston Globe discusses the importance of attic ventilation, and how it is important in both the summer and winter.
"In the summer, a good flow of ventilation will remove unwanted heat that is trapped in the attic. That heat can damage the roofing, and it also makes it that much more difficult to keep your home cool. In the winter, removing attic heat allows the underside of your roof to stay closer to the ambient temperature of the outside air, which helps prevent ice damming. And throughout the year, good attic ventilation removes excess moisture before it can accumulate and create the potential for mold growth or damage to wooden structural members."
How much ventilation does your attic need? That depends on the size of your house and, to some degree, its shape.
The article contains much more information and specifics about attic ventilation.