Stella McCartney cobalt blue shift dress

In July the Duchess wore this "cobalt" crepe dress to a pre-Olympic Games exhibition at the National Portrait Gallery in London.  The dress also mad a reappearance at a tennis match during Andy Murray's Olympic tennis match.

The shift features a high, round jewel neckline, short sleeves, and a high waistline.  I like that this dress is very flattering on her figure.  I think the neckline and sleeves help mask her wide shoulder line, and this particular shift style is just boxy enough with shape at the waist.

The length is above-the-knee, and just about thigh-high on Kate, which I think is just a tad high to be appropriate for an official duty.

The most notable detail of the dress would have to be the exposed darts along the bodice, which are on the outside, as opposed to the inside of the dress with only a seam visible.

 

The dress is by Stella McCartney and is mostly sold out, but I included links of two of the most recent listings I found.
It retails for approximately £515 to £659, or about $836 to $1069 U.S.

Stella McCartney via Selfridges - crepe dress.
Also at Matches.

For the repliKates its fairly easy to find the basic silhouette of the narrow shift dress with sleeves.
I few that I thought were similar were these two Eliza J dresses.
The one on the left is a a shift dress for plus sizes that features slimming princess seams, 3/4 inch sleeves and side slit pockets.  With the bold blue shade, the neckline, and the shift silhouette, I really feel that this dress does capture a good imitation of Kate's
It retails for approximately $132, U.S.
The dress on the right is also by Eliza J, a drop waisted sheath style with black pocket accents.  This one is a bit further from Kate's style since it is looser and has a dropped waist but still very much captures the essence of Kate's.  I think its important to be a bit flexible with look-a-likes because I want to be able to find a range of options that look like the original but that are also flexible for other body types.
This dress for about $128, U.S.

Left: Eliza J via Nordstrom - ponte knit dress Plus;

Eliza J via Nordstrom - drop waist shift dress.

This option from Dorothy Perkins is a bit different because the fabric is more textured than the original, and also has a different shape, with a natural waist and slightly gathered skirt.

Still, the neckline is round and high and it has longer length sleeves.  Its a more relaxed, casual version of the original.

This dress retails for approximately $39, U.S.

Dorothy Perkins - navy half sleeve dress.

Last but not least is this Banana Republic shift dress in black.
The sleeves are longer length and the dress has side pockets and leather trim, but the loose shift style closely resembles Kate's original.  I also think that its the closest in level of dressiness and formality to Kate's.
This dress retails for approximately $130, U.S.

Banana Republic - faux leather trim dress.