Chief Justice Roberts's interrogative "So help you God?" was not a mistake. Nor a constitutional misstep, methinks.

Up to that point he had been feeding the prescribed lines (if he'd read them correctly), which Obama was required by the Constitution to repeat.

Then, just to more or less continue the style of call and response, he prompted Obama's "So help me God" as a question, a personal prompt, but as a question (sort of, "Do you want to add 'So help me God?'", so as to distinguish it from the preceding essentially commands to echo in the previous partial lines. Roberts could have just quit, and Obama say "So help me God" on his own, but asking the prompt was a matter of style.

Obama's response was not, I believe, any violation of the Establishment clause for at least two reasons:
1. The Establishment Clause specifies something Congress may not do, and is not phrased as applying to the president.
2. Obama's expression had to do with himself, not the government, not the people, not laws of any kind, not appropriations, and not the State.

Dave Oesterreich