This post has become VERY popular recently thanks to Pinterest! Since some commenters have had some concerns about the oven cleaner, I wanted to give a little more information about the chemicals. Oven cleaner contains lye, which IS a caustic agent, typically sodium hydroxide or potassium hydroxide. The lye residue is washed off of the skillet, neutralized with an acid soak (vinegar), then washed again and coated with seasoning.Lye is used in many everyday items such as soap; it also shows up in food preparation, from olive curing, to lutefisk (eww), to making authentic pretzels. It's also used for biodiesel!
Lye is very hygroscopic, meaning it will pull moisture out of the air. This is why, when being used on grimy cast iron, the aerosol foam ends up becoming a brown liquid. It's dissolving when it contacts the air, which results in VERY little if any of the lye being absorbed into the metal.
Even groups such as the Griswold & Cast Iron Cookware Association RECOMMEND the oven cleaner method for reconditioning cast iron.
Some people advise using high heat or a self-cleaning oven cycle (which uses high heat) to clean cast iron. I shy away from this method mainly because the majority of my cast iron collection is antique. High heat CAN and WILL warp or even crack fragile cast iron. Plus, my oven doesn't have a self-cleaning cycle. I also can't simply throw my cast iron in a fire, being that I live in the suburbs, and setting yard fires is strongly frowned upon here!
*This method is for 'plain' cast iron ONLY. It is NOT for enameled cast iron such as Le Creuset. Do NOT try this with enameled cast iron.