Koran vs. Bible vs. US Constitution

There's a lot of debate about whether newly elected Congressman Keith Ellison (D-Minn)., the first Muslim elected to our Congress, should be allowed to swear his oath on a copy of the Koran, rather than the Bible.

I'm baffled as to why there is any debate at all, and questioning the accepted practice of having our public officials swear their oaths on the Bible. Are this country's foundational principles not based upon the notion of freedon of religion? Isn't there supposed to be a separation between church and state? Why are our public officials swearing their oaths on anything other than a copy of the US Constitution?