Early this year, the FASEB Journal [Federation of American Societies for Experimental Biology] reported a survey of American attitudes to evolution.
Surveys typically show a surprisingly high proportion of Americans don't believe evolution is a fact - New Scientist has in the past reported a worse result for the US than for most developed nations. However, clearly it matters how the questions are phrased, and the overall context of the survey. Drill down, and it is likely a religion-focused survey will elicit sympathetic responses, where many will be reluctant to support evolutionary theory if it means denouncing their religious beliefs. Likewise, if the questions are posited in a rationalist context, people are more likely to want to be seen to be rational, particularly if the questions aren't seen to be placed in direct opposition to their beliefs.
One example of the difference in survey methods: in the FASEB report, a simple examination of three different surveys gave three different measures of whether Americans thought evolution was guided by natural processes or by a supreme being.
Gallup: 13% natural selection vs 38% supernatural guidance
Pew: 25% vs 18%
FASEB: 36% vs 25% on one set of questions; 32% vs 21% on another
[In each case the remainder didn't accept evolution - as it was phrased. Note that Pew Reseach is not a specifically religious organisation.]
A key finding of the FASEB survey - and a central didactic point of the accompanying article - is said to be that people's support for [teaching] evolution is directly related to their level of understanding of science. Stating the obvious, true, and it doesn't address the disparity between the level of acceptance of evolution in America and other western nations. The obvious response is the difference is due to the particular sociological - political, religious, ideological - pressures in that country.
The full FASEB report here; a press report on it here.