Is this movie a tie-in to the “Avengers Initiative” alluded to in Iron Man?

Marvel has announced that a series of movies is coming out, leading up to the Avengers movie: Iron Man (2008), The Incredible Hulk (2008), Iron Man 2 (2010), Thor (2011), Captain America: The First Avenger (2011), and The Avengers (2012). There will be cameos in all of these movies from characters in the other movies to establish that these films are in the same universe.

Who are the Avengers?

At Comic-Con 2010, we were officially introduced to the full line up of Avengers. Those who will appear in the movie include Steve Rogers/Captain America (Chris Evans), Thor (Chris Hemsworth), Iron Man (Robert Downey Jr.), Bruce Banner/The Hulk (Mark Ruffalo), Clint Barton/Hawkeye (Jeremy Renner), Black Widow (Scarlett Johansson), and Col. Nick Fury (Samuel L. Jackson). AntMan, Wasp, Quicksilver and Scarlet Witch are not on the cast list.

Is this in the same universe as the rest of the Marvel Studios movies?

Yes. From now on, all Marvel Studios movies are part of the same universe, one of the many parallel universes in the Marvel Multiverse. This continuity begins with Iron Man and continues with every future Marvel movie except for those that have already been made and owned by other studios such as Spider-Man (whose movie rights belong to Sony), the X-Men and the Fantastic Four (whose movie rights belong to Fox), and a few others that have come out over the last several years. The movie rights to Iron Man, The Avengers, Thor, Nick Fury, The Hulk, Captain America, and others all belong to Marvel Studios, and because of this, the continuity is possible.

Why is Edward Norton not playing Bruce Banner/The Incredible Hulk in this film?

Negotiations between Norton and Marvel eventually broke down, and the role was instead offered to Mark Ruffalo, who accepted. Norton wrote a public response to the decision on his Facebook page, stating that he will not appearing in The Avengers, but thanked Marvel Studios for considering letting him reprise the role.