A recent study of 2011 Religiosity came out by Gallup. Here are the results:
Some of the conclusions are a bit strange, but it measures religiosity in general, so no surprise. Interestingly, from a solid apologetics perspective, the US is equally religious, but everyone worships something else. One man or woman worships comfort and relaxation over what is normal, one worships approval of friends and peers, another worships a false god of a world religion besides Yahweh.