You know, I think the no-soap rule is actually a myth.
From what I've read of this guy, he's usually very diligent in his research:
http://www.seriouseats.com/2010/06/how-to-buy-season-clean-maintain-cast-iron-pans.html
From the comments of that page:
"That's a myth, probably a hangover from when soaps used to contain lye and powerful detergents that would pit the metal. Modern soaps are very gentle, and designed to attack only grease, not the polymer build-up on a good cast iron pan. That said, it's best to avoid it in the early stages of building up your seasoning, but even then, if you need a little, it's not the end of the world as long as you carefully dry, heat, and re-oil your pan with each use."