Al Mohler recently wrote an op-ed piece for the Chicago Tribune about the murder of Dr. Tiller. Those who would liken Tiller's murderer to the heroic efforts of Bonhoeffer should read this piece carefully (not hard, it's short).
"America is not Nazi Germany. George Tiller, though bearing the blood of thousands of unborn children on his hands, was not Adolf Hitler. The murderer of Dr. George Tiller is no Dietrich Bonhoeffer. Dr. Tiller’s murderer did not serve the cause of life; he assaulted that cause at its moral core."
I also found this article by Greg Koukl (author of Tactics) somewhat helpful, though the writing isn't very good.