Yep, I just got married in Negril Thursday and carried my dress on and it was hung for me also. The airline (AA) will not make any promises, they leave it up to the crew to decide, but of course, who's going to break a brides heart and not take care of their dress for them?! Either way, there's no reason you can't carry it on, even if they won't hang it for you. The lady who did my alterations showed me the best was to roll my dress if I had to put it in the overhead. I didn't need to do that, but at least I was prepared. Maybe you could just check on that? I didn't open my garment bag until a few days after we arrived to see the condition of my dress, and I was luck, I didn't need it steamed or anything.
Good luck!!! It's a magical place to get married!
