So, my excellent trans-Atlantic flight was made even better by buying the Christmas edition of The Economist. In it there is an article entitled "The Battle of the Books," that looks at the publishing of the Bible vs. the Koran.
The article has these stats from a Gallup poll (with Economist commentary):
Americans buy more than 20m new Bibles every year to add to the four that the average American has at home... A Gallup survey found that less than half of Americans can name the first book of the Bible (Genesis), only a third know who delivered the Sermon on the Mount (Billy Graham is a popular answer) and a quarter do not know what is celebrated at Easter (the resurrection, the foundational event of Christianity). Sixty per cent cannot name half the ten commandments; 12% think Noah was married to Joan of Arc. George Gallup, a leading Evangelical as well as a premier pollster, describes America as “a nation of biblical illiterates”.
It appears the Gallup info is several years old, but probably still accurate.