> Never refer to a Dr. as Mr./Mrs./Ms. It just isn't done.

In fairness to Prof. Clark, it's not like he was pointing out that faux pas to someone who didn't know of his position. Jeff Atwood by this point would be well aware of Prof. Clark's qualifications.

I'm not going to say that Jeff deliberately downgraded Prof. Clark's title in his reply (as opposed to the entirely possible "I just call everyone Mr./Ms.") but given Joel Spolsky's comment elsewhere in this thread about "being nice" (note: direct quote! ;) I'm feeling some cognitive dissonance here.