They didn't tell me "you are now being subjected to the new millimeter wave scanner" or offer me the choice of a pat-down, they just pointed and herded my fiancee and I through the machine without any mention of what it was or that I had a choice to not go through it.
It was only after that I realized it was the new scanner I had just gone through, and I only realized that because I read sites like HN.
So, to other travellers: when you've opted-out did you basically have to halt in your tracks and demand an opt-out? I guess I naively thought opting out would be more obvious... but I was wrong.