I was fine with 550M for FF6 personally, though I don't know what the experts thought it was going to do. Perhaps the flip side to not many movies making 500M OS will be that maybe people are saving their money for the big movies come Fall? Just a theory. The one summer movie that was the biggest dissapointment for me OS wise was MoS. I thought Superman being a huge icon figure would have brought in way more than 370M.
I think Thor2, CF, and Hobbit could all make 500M this Fall OS. Hobbit being the most likely, then Thor2 from the Avengers effect and it being 3D, and then CF.