Scarlett Johansson will not be reprising her role as the Black Widow in Iron Man 3.  ScarJo debuted as Natalia Romanova (aka Black Widow) in 2010 in Iron Man 2, and has reportedly fleshed out the role considerably in Joss Whedon’s The Avengers.  But she won’t be returning in Iron Man 3 where sexy newcomer Jessica Chastain will compete with Gwyneth Paltrow for the affections of Iron Man (see "Jessica Chastain in 'Iron Man 3'").
Of course the character’s absence in Iron Man 3 doesn’t mean she won’t return to the big screen.  At this point it appears that only the apocalypse could prevent a sequel to Whedon’s Avengers, and ScarJo told ScreenRant that she has been pitching a Jason Bourne-like espionage thriller to Marvel: "I think you build this character in the hopes that the fans are like, "We want more."  I personally think that there is an awesome Black Widow movie in there, kind of a Bourne-type of thing that would take the whole comic book genre in a different direction."