Just a pure coincidence? Or maybe she also watches GG? ;)
Catherine Middleton chose this dress by Jenny Packham and looked absolutely amazing at the ARK 10th Anniversary Gala Dinner at Perk's Field on June 9, 2011 in London. Funny fact is, that Blair Waldorf wears the (almost) same dress in Gossip Girl while trying to impress the "Monaco royals" or whatever. Real life royal vs. tv almost royal. Real life wins.
all pics borrowed: cw, zimbio, jenny packham website