At least this seems to be the case, according to findings from a recent Gallup poll entitled On Darwin’s Birthday, Only 4 in 10 Believe in Evolution.
In Gallup's usual good form, the poll also included questions to uncover what major factors contributed to whether or not participants believe in the theory of evolution. The results? Well, they weren't all that surprising in retrospect: religious beliefs and level of education correlated well with whether or not participants believed in evolution, and whether or not they could associate the theory of evolution with Darwin. Interestingly, there is a short video from Gallup News from June 2007 giving the results of a similar poll showing Americans were split closer to 50-50 a couple of years ago.
For more details, check out the article - it includes informative data summaries and a more detailed discussion of the poll results. You can also find other related polls on the site, including One-Third of Americans Believe the Bible is Literally True, May 25, 2007, and Evolution, Creationism, Intelligent Design.