Before responding, I googgled "Dean single payer" and this really is a major effort. This is as stupid as Nader attacking Gore in 2000. The only thing I can think is that they are convinced that by discrediting Dean, they will gain people behind their views. The fact is though that what they really do is reduce the effectiveness of a prominent, credible person working for progressive goals.
From the google, I also found a nice Think Progress post that actually had a thoughtful statement from Dean that shows why he favors the public option over single payer. I add it because I had not seen it before, though I would bet you have. :) Whether people agree with the reasons or not, they are thoughtful and motivated by a desire to give people a system that works.
http://wonkroom.thinkprogress.org/2009/03/04/dean-single-payer/But, Dean supporters might do well to put their own house in order. The DFA site is front paging a campaign by Adam Green and Bold Progressives which is targeting conservative Democrats who are against the public option. However, they include as a target John Kerry, who has been for the public option at least since he ran for President. In response to people who have written him, Green responds that his write up acknowledges that Kerry supports the public option, but requires of Kerry, and only Kerry, a statement that no legislator would ever agree to stating categorically that he would never vote for a "watered down plan". In fact, responders on the site have uniformly read the description and attacked Kerry for NOT being an advocate of the public option. Several people, including myself, have written via the DFA site to Jim Dean and to the general site with complaints on this - pointing out that JK is on Dean's list as for the public option and has said so repeatedly - including a response he wrote on Daily Kos.
Now, I am not intending to highjack your thread, but this is a very parallel situation. The question I have is who will be the liberal voices after Kennedy? Obama can't be the only voice. Dean and Kerry are intelligent, articulate men with credibility when speaking in the mainstream. Dodd has been very good on healthcare, but he has considerable baggage at the moment that weighs him down. If the left succeeds in hurting these men, they won't be replaced in the mainstream media by Conyers and Kucinich. they will be replaced by people to their right - like Schumer and even, Conrad or Baucus.