I don't believe them.
Most people I've met would hesitate to perform the "pushing a fat man" action regardless of context. Think about what you'd have to do to physically push him onto the tracks. Make sure to picture your arms wrapped around him as you tackle him, the fabric of his tweed jacket abrading your face, your nose filled with his sweat, his cries of surprise and alarm and protestations all you can hear above the noise of the trolley as you strain against his bulk.
I believe all of that would be readily apparent in the moment, and most people would hesitate. But maybe I'm just thinking of northeasterners.
"Would push the fat man without hesitation" was lazy formulation on my part. They said without hesitation that pushing the fat man was the morally preferable choice, not that they would be able to act according to this belief.
At Reason Rally a couple of months ago, we noticed that a lot of atheists there seemed to be there for mutual support - because their own communities rejected atheists, because they felt outnumbered and threatened by their peers; the rally was a way for them to feel part of an in-group. Reason Rally is definitely an event that selects for people who feel excluded by their communities most of the time. But there may be a different concentration of people who have had this sort of experience on LessWrong, and we wondered what that concentration was.
Hence, this survey: LessWrong Members and their Local Communities.
If I get a decent sample size, I will post the data for all to enjoy.
EDIT: I added two questions about current and previous religious views to the poll. If you took it before 11:30PM EST 5/2, I'd appreciate it very much if you would take the time to retake it. :)