Via the Political Animal blog, I see that beliefnet founder Steve Waldman has written a book on the subject called Founding Faith. The book is definitely going on my reading list, and this comment only heightens my interest:
Waldman has read just about every available thing that George Washington, Thomas Jefferson, Benjamin Franklin and the rest said and wrote, publicly and privately, about their personal theological views. He comes to two conclusions. First, all the Founders saw themselves as Christians and believed that God in one way or another guides human affairs. So, score one for the religious right. Second, not a single one of the main Founders actually believed in the divinity of Jesus, which is the central tenet of the Christian faith. Score one for the secular left.