Demosthenes talks about pseudo-nymity online.
I don't have much to add to this issue, really. I'm anonymous because I worry about employment and personal consequences of what I write here. Given the excitability of certain online elements, I don't think the latter concern is that unreasonable. As for the former - maybe that's paranoia but given the long memory of Google I don't want my current and future employers being able to hold my words against me. A casual overview of what other Bloggers do tells me that many are either self-employed or otherwise have either financial/job security or careers which appear not too likely to be impacted by doing this type of thing. I don't make a living doing this, so I have to worry about that.
Having said that, I've stated before that I am not in any way a political or media insider. If I were, being anonymous might be an issue.
People are free to like it or not. It's quite silly to use it to undercut anything I say. In addition, contrary to what many in the Blogosphere like to say, there are plenty of pseudonyms on both sides of the political spectrum. People tend to ignore the issue when Bloggers they like have pseuds, and bring it up for Bloggers they don't like. Some people have no-linking (or no perma-linking) policies for "anonymous" Bloggers, which they institute on an inconsistent basis - which is fine, it's their sites.
The Washington Times recently started a regular pseudononymous column and I haven't heard much complaining. Ted Olson, with good reason, used to write with a pseud for The American Spectator, whose editor has chastized me and others for being anonymous. Well, whatever. It's a silly issue. Read my blog or don't, but I really don't care if you care that I'm anonymous. I do think it could be an issue at some point - as I said, if I were a political or media insider it'd be a fair issue. If I started publishing libelous stuff it would obviously be an issue. But, now? I don't think so.