Going to harp on again about 'Command and Control' here (it really is a brilliant book) but I personally find the mere existence of nuclear weaponry to be a terrifying thought, they certainly don't fill me with confidence.
There was an incident in the late 70's / early 80's (I think) where one...
From what I recall of 'Command and Control' (excellent book, highly recommended) there was a time that the US held the keys to all nuclear weapons whether they were theirs or not and thus "permission" would have been asked.
Though it wouldn't really have been permission it would have been...