I've seen this very advice in several places, but I'm not entirely sure what it's about. The magic of detergents is that they can break down oil and grease (mmm... emulsifiers), which lets you get the pan actually clean. I've never seen detergent actually remove the seasoning from any of my pans. I'm assuming this is because the seasoning isn't much in the way of a hydrocarbon any more. It's one form or another of pure carbon (graphite? diamond?) that's somehow bonded to the surface of the pan.

On the other hand, I'd avoid steel wool and the like because those will definitely take the seasoning off and expose the raw iron. Likewise, if you leave a cast iron pan with water in it (as I've done by mistake in the past), you end up with a pan of rusty water. It's essential to dry your pans.

One of my standard applications for my big skillet is putting it in the floor of my oven when baking bread. I'll then pour boiling water into the pan and slam the oven door shut to create a nice blast of steam. Needless to say, this wreaks havoc on the seasoning and the surface of the pan shows sometimes shows evidence of rust after the oven has cooled off. One nice pan full of bacon later, and everything seems perfectly fixed.