This is a green silk Maje dress from Diane von Furstenberg. It has a deep V-neck and a line of flat, gold-brushed buttons at the back of the dress.
This is a classic pared-down dress with interesting details at the back.
1) The Duchess of Cambridge wore it on 8th July 2011 in Los Angeles
2) Kate wore the dress again on 29th July 2011 in Edinburgh to attend Zara Philips pre wedding cocktail party.