This could be subtle subterfuge on Joss Whedon's part. No screenplay has been written for The Avengers 2 yet, which is set for release in 2015. It is possible that a lot may change between now and then. For now, though, it seems we won't be taking a trip to Planet Hulk anytime soon. The story line was previously explored in the 2010 animated movie Planet Hulk.
While the story is still being worked out for The Avengers 2, Joss Whedon teased that his follow-up will be inspired be classic, non-superhero sequels such as The Godfather: Part II and Star Wars: Episode V - The Empire Strikes Back.
The Planet Hulk story debuted in a 2006 run of The Incredible Hulk comic books, where the mighty green Avenger was banished from the group and exiled to a planet named Sakaar. While he is first enslaved by the natives, he eventually leads a rebellion against the planet's evil Red King. He later returns to Earth to get his revenge on the Avengers. Marvel's Kevin Feige has previously expressed interest in bringing this particular Hulk story to the screen, so we may see Planet Hulk sometime in the near future, no matter what Joss Whedon says. After all, this wouldn't be the first time a director has lied to fans about a big franchise. And when it comes to Marvel's The Avengers, its the biggest franchise currently going, second, only, maybe, to Star Wars (both owned by Disney).