I've never cared enough about this story to pay close attention, but for those who do, Salon has put it to rest:
We've learned, for instance, that an Associated Press reporter in Alaska who was covering Palin during her pregnancy in early 2008 (before she became a national figure) thoroughly investigated rumors that the pregnancy was a hoax. The reporter directly questioned Palin about the matter in a private meeting in her Juneau office before she gave birth. Gov. Palin responded by voluntarily lifting her outer layer of clothing, offering a clear look at her round belly. The reporter quickly concluded that there was no truth to the rumors and never wrote about them.
My understanding of this story certainly helps me understand how Bitherism persists among low-information voters. I follow politics in general very closely, but I've just never cared enough about the Trig issue to read anything about it. I'm a low-Trig-information voter. My understanding was that you have a couple people raising doubts, and a bunch of other people calling those people crazy. I suspected Trig was really her baby, but I wouldn't have staked my life on it.
I'm certainly not excusing conservatives who promote Birtherism, or even the GOP politicians who sidestep the issue ("I believe he was born in America") without refuting it as a matter of established fact. But I do understand how intelligent people who don't really pay attention to the issue could see it as some kind of hazy controversy. The responsibility to put the controversy to rest lies with elites.