According to Avengers cinematographer Seamus McGarvey, not only were parts of the movie filmed with an iPhone, they are even in the trailer:
The beauty of photography or cinema is that you make every choice based on the content at hand. On The Avengers, I did a couple of shots on the iPhone and they are in the movie. In fact, they are in the trailer! I understand that sometimes there is no choice and you have to go for the cheapest option, but if you are limited for choice, you can still make poignant decisions that will effect the look of the film
The movie is scheduled for May 4th 2012, starring Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as the Hulk, Chris Helmsworth as Thor and Scarlett Johansson as the Black Widow.