Personally I don't see the problem with Bill. Atheists are atheists and you can't expect that lack of belief in a god to encompass skepticism and the ability to see past the stupidity of every claim made by every woo enthusiast. While I'm sure many atheists are staunch skeptics, real skepticism - the ability to ask the right questions and adherence to a predominantly scientific viewpoint, are often beyond the education and ability of many people. Hence atheists who believe in woo and become anti-vaxers or truthers or birthers. Atheists are really only bound by that single lack of belief.
Given that perspective - I don't personally care what Bill thinks (but his influence makes it a shame). As the organizer of a conference, however, I would not want -any- ignorant birther, anti-vax whacko or other person spouting their inspid nonsense. Here's some of what Bill M. is said to have said:
http://skeptico.blogs.com/skeptico/2005/12/drinking_the_an.html#comments