Mark Ruffalo told NYMAG’s Vulture Blog that he is playing the Hulk AND Bruce Banner in the upcoming Avengers movie. “"I'm really excited. No one's ever played the Hulk exactly, they've always done CGI... They're going to do the Avatar stop-action, stop-motion capture. So I'll actually play the Hulk. That'll be fun.”
So essentially the same system that was used for the amazing performance capture in Avatar will be used here, If I read between the lines. And Summer 2012 is when the movie hits the big screens!!