I found this report of a Suffolk University poll (http://www.capecodtoday.com/blogs/index.php/2010/10/30/suffolk-university-poll-patrick-maintain?blog=101) putting Galvin-Keating-Henderson at 49-18-5. So on the one hand, Henderson seems like a long shot, but on the other hand, it may be safe to vote for him because Keating is also really far behind.