Is America a Christian Nation? (1)

Yesterday, addressing students in Istanbul, President Obama declared that "America is not a Christian nation."

That is a pretty bold statement especially considering it was made in a predominantly Muslim nation.

Without any ad hominem attacks on President Obama or each other I would like to know your thoughts. Specifically I would like you to deal with the question, "Is America a Christian nation?"

If your answer is "no" then what is America? If your answer is "yes" then explain. To what extent is America "Christian"?