According to PerezHilton.com, The Twilight Saga: Breaking Dawn is slated to start production when filming for the third movie wraps. This news comes from an interview with Alice actress Ashley Greene, in which she said "We're going to start right after Eclipse."
With Eclipse due for release in June 2010, we could well see Breaking Dawn before the end of Summer 2011. Talk about cashing in on the saga's success — Summit is rolling these movies out faster than the books sold on release day.