Switzgal, it isn't actually any part of the book, technically, as far as I remember. I am a new Twilighter and just read all the books for the first time including New Moon, I read it TWICE in the last month. Technically Bella didn't know it was Jacob who saved her from Laurent. She only saw the wolves come into the clearing as a pack and something about the wolf reminded her of Jacob when he looked at her. (I think I have that right, feel free to correct me).
I am so glad I came here. I was SO SO SO excited when I saw the trailer BUT after thinking about it I was struggling with things like:
1.Why isn't Laurent sparkling because the sun was out in that scene?"
2.It looked like Laurent was at La Push when he was with Bella because Jacob ran from the porch of his house to save her. That didn't happen at La Push, it happened in the meadow.
(Maybe Jacob's jumping from the porch was an edit from another part of the movie, if not, why would they do that? The vampires--not just the Cullens but really none of them--crossed the boundary line because La Push was protected by the pack. That is why the wolves exist, to protect La Push. If Laurent was at La Push than it implies that the Wolves aren't doing there job, which we know didn't happen. They always did their job.

That bugged me only because after reading these books incessantly the last month I am of the opinion that Stephenie Meyer did a perfect job of thinking of everything....I don't want them to mess it up!)
3. At the birthday scene, part of what makes it so dramatic is the reactions of everyone...not just Edward's reaction. Now maybe they didn't show everyone's reactions in the trailer but it appeared that Edward was the only who reacted and everyone else just stood there. (I could be wrong).
Stuff like that was starting to worry me only because I am SOOOOO excited for this movie and I absolutely LOVED the book! Coming here got me excited again. Reading the reactions of some of you made me realize how great it is that we get to see this story come to life no matter how it is interpreted. I can't wait. That phasing WAS AWESOME!!!