Eugene is correct about public hostility to atheists and the willingness to admit discriminatory feelings against them. In 1991, the General Social Survey asked 1244 respondents this question (variable POLSGOD):

How much do you agree or disagree with each of the following?: Politicians who do not believe in God are unfit for public office.

RESULTS:

15% Strongly Agree
15% Agree
27% Neither Agree Nor Disagree
31% Disagree
11% Strongly Disagree

Note that 30% think that atheists are "unfit for public office," and only 42% actively disagree with the statement that they are unfit.

By comparison, in the 1991 GSS, 90.5% of Americans said that they would vote for a qualified black for President if nominated by their party. Similarly, 91.4% of Americans said that they would vote for a qualified woman for President if nominated by their party. The difference for atheists is stark.

In looking at some demographic breakdowns, there is no difference in tolerance of atheists between Republicans and Democrats, but there is between conservatives and liberals, with liberals being significantly more tolerant. Also, whites are significantly more tolerant of atheists than African Americans.

All this emphasizes for me how different the law teaching world is, where atheists (such as myself) are strongly over-represented and Christians, particularly fundamentalist and evangelical Christians, are strongly under-represented.