Speaking in the following video, Ruffalo is asked about the possibility of a Hulk movie.
I think they are for the first time sort of entertaining the idea of it. When we did Avengers 1 it was basically "no." And now I think there is some consideration for it, but still nothing definitive. Not even a skeletal version of what it would be. I look forward to going down that road if we could crack the nut. I think the Banner/Hulk storyline is not an easy nut to crack. If they did Iron Man 4 and Banner is in that, that would be very cool.
"The Avengers: Age Of Ultron" hits May 1, 2015 directed by Joss Whedon starring Robert Downey Jr., Chris Evans, Chris Hemsworth, Scarlett Johansson, Mark Ruffalo, Jeremy Renner, Samuel L. Jackson, Elizabeth Olsen as Scarlet Witch, Aaron Taylor-Johnson as Quicksilver, Paul Bettany as Vision, with James Spader as Ultron.
Head on over to the Cosmic Book News Avengers movie hub for more.