I thought I'd weigh in, since I work in bridal.
First of all, let me say that I totally understand people who like sleeves of some sort. I was the same way. I have a relatively small frame but with fairly broad shoulders and I always felt I looked half-dressed in a strapless. Additionally I, like so many others, have had the bad strapless prom dress experience wherein a poorly fitted gown slides down all evening.
When I got married I was able to find an absolutely gorgeous gown with cap sleeves that I liked better than any of the strapless I had tried. They do exist, and they actually are becoming more popular. While the shop I work in has carried some very pretty keyhole back dresses for as long as I've worked there, we didn't have as many dresses with other types of strap or sleeve till recently. Over the last two seasons we've probably tripled our stock of dresses with sleeves of some form, including more than a couple with full sleeves. We've also put together a collection of boleros in various styles. All of these are very popular, though most brides still come in looking for strapless. Additionally, almost any strapless dress can have some form of strap or sleeve added, though not every bridal shop is willing to do so. We can also alter any straight across neckline into a sweetheart.
That said, a strapless gown fitted properly should not slide at all. Well, I should ammend that to read a well-made strapless gown that is fitted properly. What most people don't know is that the waist is the crucial part of the fit. No matter how perfectly the top and bust areas of the dress fit, if the waist does not fit then the dress will slide. When the waist fits, however, it holds the whole dress in place. It keeps the dress from sliding down by cinching in at the narrowest point, and it holds the top of the dress in place with the boning. Whether or not you like boning, the stuff has a purpose. It shouldn't dig into you- that's a poor fit, too- but it should fit like a second skin, albeit a second skin that is built extra tough. We've fit women with 42-F sized chests into strapless gowns that held them up and in place.
After I'd worked at the shop for a while I had a chance to pick out a discontinued bridesmaids sample on sale for a party. I tried on a strapless on a whim since it was in my size and was surprised that it worked really well. I had to take it in a bit, but the cut actually flattered me (I've found I have to really stick to certain necklines to keep my shoulders from looking disproportionately broad). I also found a strapless bra that worked much, much better than any I had ever tried before in our extra stock, and was amazed how well it worked. I'm now a convert to strapless to a degree, though I still like straps best in many cases. I did wear it with a pashmina shawl to deal with air conditioning.
I have seen the Disney styles. I have to confess, I'm not a huge fan..... usually too gaudy.
Anyways, that's the end of my long rambles on the topic. : )
