Since I wrote on this earlier in the week, I'll pass along some relevant links, and renew my previous plea to reporters and/or pollsters to investigate this more carefully. John Quiggin has a long, interesting post in which he interprets birtherism as a "shibboleth" -- I'll let you read the whole thing, but it's intriguing. Jon Chait responds by, in part, raising our old friend the closed information feedback loop. And Dave Weigel's thoughts about birtherism as a partisan marker are worth reading, too.
I should also pull up from comments tesibra's reporting that there's a large strain of birther belief that fully accepts that Barack Obama was born in Hawaii, but believes that he is nevertheless not a "natural born citizen" because of a strained reading of the Constitution -- one that has the added benefit (from one point of view) of also disqualifying many Mexican-Americans from citizenship.
I'll just mention again that while all of this is interesting, it's almost completely speculative, at least at the mass level. What is clear is that Republicans, and especially the most partisan Republicans, have learned to answer polling questions with birther answers. As far as I can tell from the discussion, however, we really know very little about what most birthers actually think beyond how they will answer those questions. As I argued, there really is a range of possible beliefs here that would generate those answers, some of which are pretty disturbing (in that imply either rabid conspiracy thinking or rabid racism or both) while others are largely benign. And again, see the links above and comments on my previous post for even more plausible speculation -- but it really is speculation, as far as I can tell.
I'm all for the kinds of interpretive analysis that Quiggin presents in his post, but I also think that there's an empirical question here once we get beyond the elite level for which more information would be very helpful, whether such information is gathered by reporters doing follow-ups to polling, or by someone (maybe these folks?) doing a more extensive survey.