If Hulk is the main villain it will be a ridiculous decision by Marvel, as they will run the characters name into the ground, the Hulk is an anti-hero, not a villain.
I wouldnt mind him 'appearing' as a villain for the first half of the movie, but during a fight the Hulk saves a bunch of innocents from a collapsing building and they realise he isnt a villain, he is just defending himself.
I am really interested with what Whedon does with the character, but I hope they aint making him the villain.