I have the same reaction I've had for some time: I really do wonder how large the subset is of those who believe that Barack Obama was born in Kenya (or elsewhere) but who also are totally unaware that there's anything important about it. I mean, I suspect if you did a poll of baseball fans I suspect that you would find a fair number of them who believed that Alex Rodriguez was born in the Dominican Republican and not, as he actually was, in New York City -- but none who believe that it matters at all, which of course it doesn't for any baseball reason. I strongly suspect that a decent-sized chunk of "birthers" are aware of neither the natural born citizen requirement nor that there's any controversy about the whole issue; they heard someplace that Obama was board abroad and he has a funny-sounding name, so they figure he was foreign-born and that's sort of the end of it, no big deal.
(And that's outside of the presumably small, but perhaps real anyway, segment of the population who may believe that someone born in Hawaii was not born in the US, including anyone who believes incorrectly that he was born before statehood and that therefore he was not born in the US).
As far as I know, we don't even have a baseline to compare it to. How many Americans believe Mitt Romney was born in the US? How many believed that John McCain was (and recall: he wasn't)? How many believe Arnold Schwarzenegger was born in the US? On each of these: how many would say "not sure"? Wouldn't knowing those answers help us better interpret supposedly birther sentiment?
Perhaps I'm wrong. It could be that a full one-fifth of the US population really believes that Obama was born in Kenya and has conspired to cover it up, producing false documents, in order to evade his Constitutional ineligibility for office, and another 25% of all Americans are hard pressed to choose between that story and the truth. I don't know! And of course it's still interesting if a lot of people have misinformation, whatever it means to them.
But I really wish we could get a bit more than we have on this one.