Update (4/7): The more the merrier! Cotillard is not set to replace Carla, but rather to act alongside her. From CNN:
Bruni is an Italian ex-model and will still be playing a part in the movie. Oscar winner Cotillard, who is French, will also have a role in the film, [Woody Allen's] rep says.
Allen said last week, "Carla Bruni is not a woman who earns her living as an actress. She is a first lady. She could be taken away at any moment by other duties, like a political crisis or an important event. I have to take account of the risks that there could be, and hiring her is far from certain."
Now, Allen confirmed the Cotillard will take the leading role. They'll start filming in Paris in July.