Jessica Chastain said yesterday that she passed on the Iron Man 3 role, and it looks like they've already found a replacement and its none other than Rebecca Hall, (The Prestige, Vicky Cristina Barcelona, The Town, Everything Must Go).
Variety reports that she'll be playing a scientist who plays a pivotal role in the creation of nanotechnology which becomes a weapon sold to terrorists.
This casting confirms that Hall will likely be playing Maya Hansen, a scientist who works alongside Aldrich Killian, being played by Guy Pearce. They develop "nanotechnology, known as Extremis, that winds up being sold to terrorists." She has a history with Stark having studied with him in college, so it stands to reason that she could create a bit of a love triangle with Tony and Pepper Potts (Gwyneth Paltrow).