Salon had a good article explaining the psychology behind the Nirthers:

Evan Harrington, a social psychologist who is an associate professor at the Chicago School of Professional Psychology, agrees. “One of the tendencies of the conspiracy notion, the whole appeal, is that a lot of the information the believer has is secret or special,” Harrington says. “The real evidence is out there, [and] you can give them all this evidence, but they’ll have convenient ways to discredit [it].”

Whatever can’t be ignored can be twisted to fit into the narrative; every new disclosure of something that should, by rights, end the controversy only opens up new questions, identifies new plotters. Perhaps the most common argument of those questioning Obama’s eligibility is that he should just release his full, original birth certificate, rather than the shorter certification, which is a copy. His failure to do so only proves there is reason to be suspicious, they say, and if the document was released, the issue would go away. But that’s unlikely. It was, after all, the Obama campaign’s release of the certification this summer that stoked the fever of conspiracy mongers.

For believers, it works like this: So what if Dr. Chiyome Fukino, the director of Hawaii’s Department of Health, released a statement saying she has verified that the state has the original birth certificate on record? So what if she said separately that the certification looks identical to one she was issued for her own Hawaii birth certificate? Why didn’t her statement specify Obama’s birthplace? So what if a Hawaii Health Department spokeswoman later clarified that Fukino meant that Obama was born in Hawaii? So what if researchers for factcheck.org actually saw the physical copy of the certification and debunked much of the key “evidence” supposedly proving that the image posted online is a forgery? They’re not really independent. They’re funded by the Annenberg Public Policy Center, and Obama once (with Bill Ayers, no less) ran an entirely unrelated program that happened to be paid for with money donated by Walter Annenberg. And on and on and on.