This is why you can use soap on cast iron, myth # 4: "In a properly seasoned cast iron pan, one that has been rubbed with oil and heated repeatedly, the oil has already broken down into a plastic-like substance that has bonded to the surface of the metal. This is what gives well-seasoned cast iron its non-stick properties, and as the material is no longer actually an oil, the surfactants in dish soap should not affect it. Go ahead and soap it up and scrub it out."
I generally just wipe mine out, or scrub with coarse salt. I've only had to use soap twice when it just refused to come clean with anything else. In those cases I was really thorough about drying and reoiling.