Why is President-elect Obama's selection of Christian Rick Warren to deliver prayer causing controversy ?

Rev. Rick Warren's choice to deliver a prayer for the inauguration seems appropriate to me. He seems to be a very sincere man, who has written an excellent book, "The Purpose Driven Life," He has made good sermons about not being overly concerned with materialism in contrast to the prosperity ministry. Surely people will recognize as a Christian he would likely invoke Jesus' name. Cannot people separate acts of policy and a simple prayer? Would not such a watered down prayer be lacking? Society is tolerate of many things such as dirty t.v. shows and movies, cannot the same be exercised in an important formal occasion?