If a couple are both into the big, dramatic public proposal, that's fine for them; I would never want one myself, nor do I particularly enjoy watching them. I voted I would say no, if my answer was no. And I agree with those who said that even if my answer might not have been no before that point, surprising me in so public a manner would show the person did not really know me very well at all, and they would get downgraded to at best a "let's talk about this later."
I am of the opinion that if two people have discussed marriage and agreed to marry each other, they are now engaged and there is no need for any kind of "one moment in time" proposal event, whether public or private. I should say, this is my opinion for me. For other people, it's really no skin off my nose if they want to do it differently. Privately I think it's a bit silly that my friend and her SO discussed marriage, agreed to it, and picked out a ring--but she didn't consider herself "engaged" until he actually presented her with the ring, as an "event" that she could then tell people about. BUT, it is totally not my business and I would never tell her I thought what she did was silly, because that's just rude. Let her have what she enjoys. Same with the jumbotron people. I feel very strongly about it for myself, but I also feel very strongly about not telling other people what to do in a harmless situation.