Marvel's President of Production Kevin Feige has previously stated that he'd like to see The Hulk in The Avengers and mentioned at the San Diego Comic-Con that "it's safe to assume that there will be some members of the Hulk universe" in the movie as well. So if Hulk is to appear in the The Avengers, does that mean Edward Norton will reprise the role? Norton is keeping things mysterious:
I won't comment on that, just because they [Marvel] keep a pretty tight reign on what they are letting out. I'll let them say.
The Hulk is also scheduled to appear briefly in Iron Man 2, much like Robert Downey Jr. showed up in The Incredible Hulk. Norton is signed to do another Hulk, though whether that means he will also be in The Avengers is something only Marvel can say for sure.