If memory serves me right, yes it is. She reportedly said it when she was told, by her advisers, that the people had no bread. This was just before the French revolution, during a famine.
There seem to be mixed opinions about this quote. Lady Antonia Fraser has written a biography of Marie Antoinette and insists that she never said, "Let them eat cake." Click on here for more:
http://www.ananova.com/entertainment/story/sm_657574.html
Marie Antoinette might have said "Let them eat brioche" A very sweet bread. Other countries didn't mix sweet & savoury so cake ended up in the translation.
Actually, it is my understanding that Madam Campan, the Queen's most trusted advisor, said it. She meant to say to give the people the flour used to make cake so they would at least have something to eat. Naturally, because the people hated the Queen, the quote was used as another reason to send her to Madame Guillotine...