I, too, love to play with proportion, and I really like the 2 Preens, the 2 Balenciagas and the white Chloe. The brown jumper-ish Chloe isn't much my style...I tend to look unbalanced in especially sweet/school-girl type styles.
The two Sienna pics are not that far off from current mainstream proportions, so I agree that some of this style will definitely be entering mainstream. But, at least where I live, I don't see the Balenciaga, Preen and Chloe shapes going very far. Mainstream dresses are nearly always about BLATANT sex appeal—some part of the dress must be very formfitting, whether that material is fitting around the bust, stomach, posterior and/or legs. These loose dresses often don't contour to ANY part of the body and will thus be less likely to appeal to a mass audience, imo.
I like these dresses because it takes a woman with confidence and spice (or, of course, a model's body) to wear a dress that doesn't obviously flaunt all of her fabulous assets—whatever size those assets may be.