I think that all the phrase "so help me God" means in common usage when most people us it is this: "I want to particularly stress and emphasize in the deepest seriousness that I DID mean what I just said."

So lighten up, guys. It's not a big deal. If I was taking the presidential oath, I'd deliberately say it just to bug the lot of you! ;-) (but not because I am one bit worried about God's part in the whole thing)