The first film was based out of the Playa del Rey facilities in Los Angeles, while the second shot principally at the Manhattan Beach Studios that will be the home to James Cameron's two Avatar sequels.  An anonymous source within Marvel reportedly informed LatinoReview, however, "it's 90% likely" that Iron Man 3 will film mostly in Wilmington, North Carolina.  

If production does end up taking place in Wilmington, it would encompass ten stages at the Screen Gems studios in the city.  Wilmington was previously home to Teenage Mutant Ninja Turtles, The Crow, Super Mario Bros., and The Jackal.  Currently, the television series Eastbound & Down and One Tree Hill are both based in Wilmington.

Shane Black, who was once the highest paid screenwriter in Hollywood on account of his Lethal Weapon, The Last Boy Scout, and The Long Kiss Goodnight scripts, will contribute to the Iron Man 3 screenplay.  The primary writer on the sequel is Drew Pearce, who also wrote the Marvel adaptation Runaways.  Though ready to go, Marvel Studios postponed that project in order to focus more completely on The Avengers, which just concluded principal photography under the direction of Joss Whedon.  The second Iron Man sequel will be the first Marvel movie post-The Avengers, in which old shellhead will fight alongside Captain America, Thor, and the Incredible Hulk.

Right now, Robert Downey Jr. is the only castmember confirmed to return for Iron Man 3, though it's extremely unlikely that Gwyneth Paltrow and Don Cheadle won't be back, as well.  Favreau is now directing Disney's Magic Kingdom, but he has repeatedly joked that he would be happy to return as Tony Starks chauffeur Happy Hogan, and I hope Black takes him up on it.