Yes, it is. Many people try to pull the old "the Constitution doesn't say 'separation of church and state' anywhere" argument, but that just doesn't hold water. True, those exact words do not appear in the document; however, their intent does.
Let's start with what the First Amendment actually does say, shall we? "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." The first part is known as the Establishment Clause, and it is the meat of the matter. What exactly does it mean to respect the establishment of religion?
Rather than put it in my own words and have you accuse me of being a Communist-Manifesto reader (which, by the way, I am), allow me to quote the Supreme Court. In the Court's decision on Engel v. Vitale, 1962, Justice Black said:
When the power, prestige, and financial support of government is placed behind a particular religious belief, the indirect coercive pressure upon religious minorities to conform to the prevailing officially approved religion is plain.
Call me crazy, but I think having Congress and the President host the National Prayer Breakfast is a pretty clear example of throwing the power and prestige of the Government behind Christianity. As is the use of the title "National" for this event; everyone knows that this makes it sound like an officially-sanctioned event sponsored by the Government. Oh wait, it is.
But Justice Black didn't stop there. Of the purpose of the Establishment Clause, he said:
It's first and most immediate purpose rested on the belief that a union of government and religion tends to destroy government and destroy religion....The Establishment Clause thus stands as an expression of principle on the part of the Founders of our Constitution that religion is too personal, too sacred, too holy to permit its "unhallowed perversion" by a civil magistrate.
Oooo, now that's unexpected! The Founders chose to separate these two not to protect laws from religion but keep religion unsullied by government! That's not an argument you hear every day. Frankly, I think it's a bit grandiose of a statement, but the core ideology is nonetheless intact: mixing these two institutions has historically proven detrimental sooner or later.
But wait, there's more:
Another purpose of the Establishment Clause rrested upon an awareness of the historical fact that governmentally established religions and religious persecutions go hand in hand.
Case in point: the furor over the Non-Mosque Not at Ground Zero. If there were not such widespread misconception that we are a "Christian nation," would there be such oppression of Muslims? Another case in point: I heard some people expressing outrage that Muslims were allowed to pray on the grounds of the White House. Upon questioning, it became patently clear that this indignation was not over the act of praying at the W.H. but that it was a Muslim prayer; expression of Christianity would have been a-ok. Long story short, I caught them in their own logic: anyone truly believing in freedom of religion wouldn't have batted an eyelash at this occurrence, and it was only because of a perceived State religion that these people were offended.
Now, for those of you who may consider Justice Black a Communist, allow me to quote from the Burger Court. In Lemon v. Kurtzman (1970), this Court developed a three-part test for determining establishment of religion: (1)whether the program at issue has secular purpose; (2)if the primary effect is neither to advance nor inhibit religion; and (3) whether the legislation fosters "an excessive government entanglement with religion."
Call me crazy again, but I'm pretty sure the National Prayer Breakfast could be considered excessive entanglement. And it certainly advances Christian dogma. We all know damned well that if the National Prayer Breakfast were sponsored by, say, Sikhs, the Tea Baggers would be up in arms over Obama and his socialist government forcing Hinduism down our throats. If the shoe fits.....
The fact that a measely two Supreme Court cases so clearly demonstrate that the Establishment Clause does in fact constitute separation of church and state should give you an idea of what the rest of the voluminous case law says. I will continue reading it, so you don't have to. Just check back here for more.