Okay, Jim's got me pulling out my textbooks. Jane, I share your interest also in the "mechanism for seasoning" and I hope what I found is helpful. Jim, Marvin, CB, Yair, please try to make sense of these passages and explain them to me!
I found this information in a book Alton Brown often holds during his show, titled On Food and Cooking, The Science and Lore of the Kitchen by Harold McGee. (Jean, A.B. IS a God!)
"...In order to forestall rusting, cooks try to build up an artificial protective layer on cast-iron pans by "seasoning" them, or coating them with cooking oil and heating them up for several hours. The oil penetrates into the pores and fissures of the metal, sealing it from the attack of air and water. There may also be an effect analogous to the behavior of the drying oils, which are largely unsaturated, prone to oxidation, and which polymerize to form a dry, hard layer when exposed to air. Since both heat and metallic ions accelerate fat oxidation, it may be that a similar layer is produced during seasoning with cooking oils....atmospheric oxygen ...is one of the major causes of flavor deterioration - rancidity - in foods containing fats. How this fat oxidation occurs is not fully known. It is thought that the electron-hungry oxygen replaces the hydrogen on a carbon adjacent to the double bond, thereby forming a very unstable complex which decomposes and produces various other reactive compounds...(Jim, help)...the more unsaturated the fat, the more prone it is to this sort of deterioration...it turns out that the tendency for highly unsaturated fats to be quickly oxidized can be very useful. The so-called drying oils - linseed oil is a common one - are used in paints and furniture polishes precisely for this quality. These materials oxidize so rapidly and completely on contact with the air that they form a dry, hard, protective film of hydrocarbon polymers."
I