I don't usually listen to the White Horse Inn. I happened to come across this episode regarding Christianity and politics (link).
Charles Hodge apparently said that having an American flag in one's church is the equivalent of singing the Star Spangled Banner at the administration of the Lord's Supper!
I enjoyed the presentation, but I found a theme that seemed odd. It seemed that the participants were suggesting that preachers cannot speak to issues of sin with respect to the political sphere. It almost sounds as though they are saying that the church can only talk about the gospel, and not the law.
On the other hand, Scripture seems to be chock full of admonitions to kings and those in authority. So, while I appreciate their concern for a clear demarcation of the roles of church and state, I think it's not quite as clear-cut as they may like.
One example I typically use is this: very often churches own property. When this is the case, they tend to get advised (more or less authoritatively) by the state with respect to temporal things, such as whether they have to pay taxes on the property, how they can use their property (zoning laws and the like), and who owns the property (if there is a dispute). It's very rare to hear that this is not proper.
If so, why shouldn't it be proper for the church to be advising the state in things moral? Why should the church-state relation be one in which the state tells the church what to do, but the church doesn't tell the state what to do?