The last time I read anything about Hulk being in the film it was about how if Ed Norton wasn't signed on to play Hulk the creators would just leave Hulk in Hulk-form the whole movie. What an awful idea. And Ed Norton won't sign on because he knows it's going to be an awful movie.
Norton wanted a hand in writing The Avengers; the studio said no way and he quit. He's also notorious for being difficult to work with...
On a side note, he was a great Bruce Banner.