I think it was actually Rousseau who said it first (at least the first documented).
Our old pal Cecil Adams of The Straight Dope explains the quotation was first written by French philosopher Jean-Jacques Rousseau in Confessions. Actually, Rousseau wrote "Qu'ils mangent de la brioche," which essentially means "let them eat a type of egg-based bread" (not quite cake, but still a bit extravagant). Rousseau claimed that "a great princess" told the peasants to eat cake/brioche when she heard they had no bread.
But Rousseau wrote this in early 1766, when Marie Antoinette was only 10 years old, still living in her native Austria and not yet married to King Louis XVI. So it's highly unlikely that Marie uttered the pompous phrase. Perhaps Rousseau invented them to illustrate the divide between royalty and the poor -- which is certainly how the phrase has been used ever since. -http://ask.yahoo.com/ask/20021122.html