[E]xploding someone's cover and revealing his identity breaches no ethical norm I can think of. Blevins had his reasons for writing as "Publius," but Ed had no obligation to respect those reasons, and he didn't have to catch Blevins in any form of unethical argumentation in order to "out" him. I'm sympathetic to Ed's view that Blevins "smeared" him, but I haven't weighed that matter very carefully, and I don't think it counts for much on the ethical scorecard one way or the other. Ed reported a fact he had a right to tell, for whatever reason it suited him to tell it—including no particular reason at all other than that he found it useful at the moment he did it.
The claim here, that we have "no obligation to respect" the decisions of our fellow human beings; that one's choice over how to present oneself in the public eye can be swatted aside for "no particular reason at all", is astounding to me. It represents one of the attributes I find most distasteful about the modern conservative movement -- the complete dismissal of interconnectedness, solidarity, or even respectful interaction. Anything I have the "right" to do, I should be able to do without criticism or compunction. Free speech means I can make a racist joke about a co-worker. Should I? Who cares -- it's free speech! The mechanics of the law and the marketplace mean that I can make billions of dollars destroying ecosystems and horde the entire thing, spending nary a penny on the public good. Should I? Who cares -- it's capitalism! Freedom is understood to mean not just legal autonomy, but moral autonomy as well -- a kind of sick Nietzcheanism that renders basic ethical interaction superfluous. For those of us who are not only for ourselves, this sort of mentality can't help but repulse.
And that's as a general matter. On the specific point of pseudonyms, Hilzoy's post on the matter shows how spectacularly irresponsible this is. Franck admits he wouldn't out a Chinese dissident who'd be tortured if his identity was revealed. But Whelan had no idea why publius chose to remain pseudonymous. His reasons turned out to be to avoid family strife, to protect the careers of (Republican) operatives in his family, and to avoid any tenure controversies. All solid enough reasons for me. But they could have been considerably more dramatic. He might have had a stalker. He might have a boss who swore never to hire Democratic scum (unlikely given that he was an academic, but a possibility for bloggers in fields without strong norms of academic freedom). The point is, we didn't know. And in absence of that knowledge, Whelan cavalierly decided to roll dice with publius' life. Because he had the right to. And that's all that matters.