But actually, damage is far more likely to occur because of exposure to water. Yup, water -- not soap -- will destroy your prized cast-iron cookware.
That's not to say that you shouldn't use water at all! You should. But, once the pan has been cleaned of bits of food and grease, the best and most important thing you can do for the health and good looks of your cast iron is to dry it off immediately. Cast-iron pans, even the most artistically seasoned ones, will develop rust at an alarmingly fast rate.
So! Dry your cast iron exceedingly well after cleaning -- that's your Big Rule.
If you really want to ensure you're doing it right -- and you should -- after drying, pour a small amount of oil for seasoning cast iron into the pan (Cook's Illustrated likes flaxseed oil, Lodge brand recommends vegetable oil), rub it in vigorously with a paper towel until the pan appears to be dry, and then set it on a medium flame for five minutes or so. That quick mini-seasoning effort will help to dry the pan, as well as bolster the layer of seasoning which, in turn, will help to prevent moisture in the air from causing the pan to rust.