Over on the French Culinary Institute's Cooking Issues blog, Dave Arnold has created a great cast iron–cooking primer that explains the science behind the cookware, including this great bit that dispels the notion of even heating:
The popular wisdom that cast iron cookware provides even heat is misleading. A cast iron skillet placed on a gas burner will develop distinct hot spots where the flame touches the pan. If you heat the center of a cast iron pan, you will find that the heat travels slowly toward the pan's edge, with a significant temperature gradient between the center and the edge. The pan will heat very unevenly because cast iron is a relatively poor heat conductor compared to materials like aluminum and copper. An aluminum pan will heat more evenly because heat travels quickly across aluminum. Because of poor heat conduction, undersized burners are incompatible with cast iron cooking. The edges of a large cast iron pan will never get hot on a tiny burner. On properly sized burners you can minimize hot spots by heating slowly, but the best way to evenly heat cast iron is in the oven.
In the photos above, Arnold demonstrates the heating properties of cast iron (top) vs aluminum (bottom) by sprinkling flour on the surface and taking time-lapse photos as the flour browns.