According to US Air, the dress is considered a carry on. My dress was my first item, and then instead of only having a purse; I put my purse inside my Disney Tote Bag ~ so that way I had my purse, and my plane necessities (snacks, book, travel folder, etc)
Also, when I got to the gate, I asked (very nicely) if I could pre-board because I had my wedding dress, and wanted my own overhead bin to share with my family only. The stewardess was very nice and said it was fine with her. So we boarded early, folded my dress bag in half (I flatened out as much air as I could before we got to the airport) and I put it in my overhead bin. Then my DH and the rest of my family put anything they needed to in there(under my dress) and then we closed it. I then made my DH in charge of the bin ~ if anyone opened it, he was to attack! LOL
Sorry for the ramble .. but HTH!