
I have the arch-insider fashion site The Fashion Spot to thank twice over today. Not only did their forum lend a definitive ID of the suit at left; it also provided a link to one of the suit's immediate descendants--a more figure-friendly variation that Neiman Marcus brought over to introduce Balenciaga to the wider American world. More about that in a bit.
The original, left, was photographed for Vogue in 1950, by Irving Penn. It is informally called 'the wishbone suit' due to the pattern of its buttons, which emphasizes its hourglass form. The model, Colette, herself had prominent hipbones and for this reason was said to be Balenciaga's favorite armature for his sculptural forms.

One final image, from the Texas Fashion Collection at the North Texas University Fashion Archives (via surgirl at photobucket via The Fashion Spot)--the wishbone suit in a new incarnation: in moire satin, with a wider skirt, bearing a Neiman Marcus label in addition to that of Balenciaga.
The department store was visionary in its pursuit of Paris couture in the 1950s, mounting a regular French Fashion Exhibition and contracting with the Parisian houses to send models over for elite clients. This fascinating history is meticulously detailed in Myra Walker's Balenciaga and His Legacy.
(I'm convinced that this gorgeous book brings as much pleasure as any garment possibly could: if you love fashion, get your hands on a copy as soon as possible.)
(Model with dog by Richard Avedon, for Vogue, November 1950 © 1950 The Richard Avedon Foundation)