Avengers Movie Hulk

Kevin Feige revealed that Hulk would show up in the upcoming Avengers movie:

"My only concern is that when we launch a franchise–whichever character’s franchise that may be–it should stand on its own two feet… So by the time ‘The Avengers’ comes in 2012, it’s not just a team superhero movie with a bunch of characters with powers. It’s three people – four including the Hulk; five including Nick Fury – who you’ve seen before in other movies, coming together for the very first time.
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It's certainly our intention to use the same actors from film to film where we can."

Marvel Studios President Kevin Feige

He doesn't tell if Hulk will be friend or foe, but my bet is, like most fans, that Hulk will be the main villain in the Avengers movie: a real showdown opposing the Avengers (Thor, Iron Man, and Captain America) to Hulk!

If Hulk does show up, Edward Norton is likely to reprise his role.
:)