Chapter of the book, For the Glory of God: How Monotheism Led to Reformations, Science, Witch-Hunts, and the End of Slavery, deals with science and religion. The subject of the religious faith of scientists comes up, and here is some data I gathered. The first table is the from the book, while the second two I cobbled together from internet sources since Nature's website seems to be down.

American Academics, Source: Calculated from the Carnegie Commission Survey of 60,028 American Academics

American Natural Scientists - 1996 Source

Prominent American Natural Scientists - N.A.S. members (1998, sample size ~ 250) and "eminent" scientists in 1914 & 1933 Source

I'll have plenty to say about science and religion soon...but please note something interesting. All the surveys seem to indicate that Mathematicians are the most religious. And as you see, the "softer" and less g loaded fields tend have fewer believers.

Of course this holds for American scientists only. From the European scientists I've met, they are probably more secular. On the other hand, I suspect that Middle Eastern and South Asian scientists tend to be more religious than American scientists....