Public Policy Polling finds some evidence the Roger Stone theory of birtherism as a base driver:
If Trump actually run 21% of New Hampshire GOP voters say they'd vote for him, compared to 27% for Romney. The key to Trump's relatively strong showing? He does well with birthers and Tea Partiers, two groups he has seemed to actively court with his public comments of late. 42% of primary voters firmly say they do not believe Barack Obama was born in the United States to 35% who believe that he was and 23% who aren't sure. Trump leads Romney 22-21 with the birther crowd, but Romney holds the overall lead because he's up by a much wider margin with the folks who dismiss the birther theory.
Now: That might actually be as far as birtherism can take a candidate. New Hampshire's electorate in 2012 is going to be thick with independents, who don't have a Democratic contest distracting them.