I think we can now be sure that Sirius is not coming back. There was the possibility that, due to the way he "died" (in the book), it might be possible that he didn't really die. In the book, Bellatrix first tried to hit Sirius with a "jet of red light". "Harry saw Sirius duck Bellatrix's jet of red light: He was laughing at her." Then (from the book) - "The second jet of light hit him squarely on the chest. The laughter had not quite died from his face, but his eyes widened in shock." It could be argued that the second spell was Avada Kedavra as it doesn't say what color the jet of light was. However, one could also argue that it wasn't the Avada Kedavra curse. 1) If it were, why didn't Bellatrix just use the Avada Kedavra curse in the first place? (As opposed to the "jet of red light". 2) I could be wrong, but I don't believe we've ever seen someone take an "age to fall" (as Sirius did in the book), nor has anyone's body "curved in a graceful arc" (as Sirius's did in the book) after being hit with the Avada Kedavra curse. Lupin was certain that Sirius had died, but it was never really made clear to the readers. I could be wrong, but it seems that, regarding what goes on behind the veil, it's still a bit of a mystery.
Anyway, now that we've seen Sirius get hit with the Avada Kedavra curse in the movie, I don't know how they could now bring him back. Jo has made it very clear that, even in the wizarding world, once a person is dead, they can't be brought back. I'm a little bit surprised that she didn't have them change this for the movie. Maybe she figured that all of the speculation will soon be over anyway, so it didn't really matter. Unless I'm missing something, it just seems to seal Sirius's fate. Not that I really thought he was coming back anyway, but I at least thought it was a possibility.