Check it out! lets you design your own dress (from a fairly broad template of silhouettes, necklines, sleeve lengths, and skirt lengths), choose your fabrics, and enter in your measurements! You can even ask for pockets! Then they custom-make your dress and send it to you in three weeks. The whole thing costs $189.
I've seen this sort of thing for bridesmaids' dresses before–in fact the models here look fairly bridesmaidy–but expanding to day dresses is a great idea.
Okay, so I have a few caveats: first, I see that you can pick some fabrics that aren't really great for the style of dress. For instance, I could have had this one done up in silk chiffon, which I think would have looked like crap — not enough body, and no mention was made of lining when I chose it. It would also be better if you could send them your own fabric, although I'm sure it would be a pain to figure out yardage for individual sizes. And if this were my site, I would have a page that shows the best dresses for certain body types: apples vs. pears, tall vs. short, busty vs. not-so, with both line drawings of the different types and real people wearing the dresses.
Other than that, though — this looks great for people who are hard to fit and don't sew themselves. The interface is intuitive and, unlike some other design-it-yourself sites, it didn't force me to use IE, nor did it assault me with a three-minute Flash animation.
The dress on the left is their "Christine," with the vee-neck changed to a scoop. I have made about ten dresses pretty much exactly like this, only with fuller skirts (and no belt) and different neckline variations, and I wear them constantly in the summer. In fact, I'm planning a white eyelet version for this summer …