At the Visionaries panel at Comic-Con, Joss Whedon announced that he is officially directing The Avengers "because Marvel can't afford the press release".
As for how he's thinking of approaching the movie, Joss had this to say:
I'm still writing the outline. I am at the stage where I am reworking and reworking. What I love about this is that there is no way that these people should be in the same room together, let alone the same team. And that, to me, is the definition of family.