I find it very interesting how nobody seems to bring up how so many Marvel movies were delayed as well.
Check out their official press release from March 2009.
http://marvel.com/news/movies/2009/3/12/7214/marvel_movie_update_new_release_schedule
It states that several of their films were being delayed from the original dates they announced back in 2008.
THOR was bumped from July 16, 2010 to May 20, 2011, roughly 10 months,
THE FIRST AVENGER: CAPTAIN AMERICA (as it was called back then) was delayed from May 6, 2011 to July 22, 2011,
and THE AVENGERS was delayed from July 15, 2011 to May 4, 2012, roughly 10 months as well.
More recently with Disney...Thor: The Dark World was delayed by 4 months (and the original director left with less than two years to the original release), Star Wars Episode 7 was delayed by 6 months due to script rewrites, and Pirates V was delayed by a year.
Just something to keep in mind.
EDIT: Before you argue that Iron Man's runaway success lead them to fast-tracking Iron Man 2 instead, note that Marvel announced Iron Man 2 on the same day as the release dates for Thor, Cap, and Avengers (May 5, 2008, immediately after Iron Man's opening weekend). The only thing that changed was that Thor was not ready.
EDIT2: Before you argue that the writer's strike justifies the change of release date, note that Marvel announced the release dates for these movies in May 2008, months after the writer's strike had ended. So, if they had no working scripts, they decided to announce the release dates anyways.