The story is actually stranger than that.
They filmed many different quips for Robert Downey Jr to say when Iron Man recovers from making a crater in Manhattan after saving the earth. Joss Wheddon picked one in the edits but when he saw the movie at the screenings for the critics, he realized his choice fell flat.
So what's an award winning director to do? Re-edit the film? Where's the spark in that!
"Reassemble" the Avengers cast and reshoot? Not a chance the studio would foot the bill for that.
But what if the cast was already going to get together -- like for the Hollywood premier of the movie.
So Wheddon conspired to have all the superheroes meet at an LA dinner the night before the Hollywood premier to shoot one last last scene. The new scene didn't make it in the overseas release since the movie was already showing in europe by that time.
It was all suppose to be a secret mission until Downey Jr spoiled it in pure Tony Stark fashion during the press conference after the Hollywood premier.