I certainly have heard about what Vincent is describing before. Specifically, I'm thinking of several incidents involving visiting faculty--usually from Europe--who offhandedly said something to the effect of "Well, I'm an atheist, so I really don't care about these things" (to students, neighbors, whatever) in response to some discussion concerning religion they'd found themselves entangled in, then were startled to discover they'd clearly said something extremely unwelcome. It's not, of course, that people suddenly start pelting them with rocks and chase them into the street--more the sort of thing you sometimes encounter traveling abroad, where a comment you assumed was totally neutral and innocent clearly somehow gave offense, and you're nonplussed by that result perhaps to the point of feeling a bit hurt that whomever you're talking to doesn't know you well enough by now to see that you're a considerate, thoroughly nonmalicious sort. While I do think the generally more religious and culturally conservative nature of American society has something to do with this, and that the increased awareness of atheism via the not-always-flattering example of Hitchens, Dawkins et al. doesn't help, I also think the many decades of the 20th century Americans spent marinating in Cold War rhetoric relentlessly associating atheism with Communism, imminent threats to national integrity, and all-around menacing 'subversiveness' might play an even bigger role. That predisposes many to perceive atheism as an inherently hostile, spit-in-your-face, unnervingly 'foreign' proposition; an attack on or rejection of community somehow, and not simply a theological stance. Hence the numerous polls demonstrating that Americans are more accepting of the idea of a woman, African-American, bachelor, Jew, Muslim, or gay man in the White House than a Nice White Married Atheist Guy.
Religious exclusivism is something quite different; as a Jew I've certainly been told numerous times that I'm going to hell for "rejecting Christ" or various more benign variations thereof, but the reaction I was describing above comes from a different place than that; it's more about implied underlying understandings of community, who's a citizen in good standing and who isn't, rather than a theological insistence.
