Joss Whedon recently put those rumors to rest, saying this.
Today, Mark Ruffalo also responded to these rumors, on Twitter, and stated that nothing is in the works as far as a Hulk standalone movie is concerned, backing up Joss Whedon's statement that there would be no Planet Hulk movie. Not in the immediate future anyway.
A lot of folks have been asking about the Next Hulk. The next time you see my Hulk it will be in the Avengers2. No plans for stand alone.— Mark Ruffalo (@Mruff221) March 7, 2013
He went on to say that he was not going to give up on the standalone features, though.
I am not giving up on another stand alone HULK. But it’s not in the works right now. One never knows what the future will bring.— Mark Ruffalo (@Mruff221) March 7, 2013
Avengers: Age of Ultron comes to theaters May 1st, 2015 and stars Chris Hemsworth, Jeremy Renner, Robert Downey Jr., Scarlett Johansson, Chris Evans, Samuel L. Jackson, Cobie Smulders, Mark Ruffalo. The film is directed by Joss Whedon.