Found on Digby (and his link to the original article doesn't work):
From a telephone sampling of more than 2,000 households, university researchers found that Americans rate atheists below Muslims, recent immigrants, gays and lesbians and other minority groups in "sharing their vision of American society." Atheists are also the minority group most Americans are least willing to allow their children to marry.
Even though atheists are few in number, not formally organized and relatively hard to publicly identify, they are seen as a threat to the American way of life by a large portion of the American public. "Atheists, who account for about 3 percent of the U.S. population, offer a glaring exception to the rule of increasing social tolerance over the last 30 years," says Penny Edgell, associate sociology professor and the study's lead researcher.
Edgell also argues that today's atheists play the role that Catholics, Jews and communists have played in the past-they offer a symbolic moral boundary to membership in American society. "It seems most Americans believe that diversity is fine, as long as every one shares a common 'core' of values that make them trustworthy-and in America, that 'core' has historically been religious," says Edgell. Many of the study's respondents associated atheism with an array of moral indiscretions ranging from criminal behavior to rampant materialism and cultural elitism.