“[…]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.”
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.