I am not an expert on polls, so if the Harris poll is flawed--let those who can demonstrate that to be the case do so. Otherwise, this is not the first time polling has yielded some pretty unsettling results about what some folks choose to believe about our president, whether it's that he's a closet Muslim (not that there's anything wrong with that) or a Manchurian candidate (something very wrong with that) or that he's not a US citizen, etc. I think because there are a number of "notable" voices that have raised questions in this regard (Limbaugh, Coulter, etc) there has been more of a tendency to see poll numbers that reflect what some of us see as an alarming willingness to entertain absurd beliefs.
Although I reside in the "liberal" Northeast, I am personally acquainted with a number of folks who truly question whether Obama is a US citizen or a Muslim. So I don't find it too surprising (although depressing) that a not insubstantial percentage of those on the Right might hold the type of beliefs referenced in the Harris poll.