Stephenie has just added this to her site:

Good news about New Moon from director Chris Weitz:

“I’m very happy to announce that Taylor Lautner will be playing Jacob Black in New Moon and that he’s doing so with the enthusiastic support of Summit Entertainment, the producers, and Stephenie Meyer.

The characters in Stephenie’s books go through extraordinary changes of circumstance and also appearance; so it is not surprising that there has been speculation about whether the same actor would portray a character who changes in so many surprising ways throughout the series. But it was my first instinct that Taylor was, is, and should be Jacob, and that the books would be best served by the actor who is emotionally right for the part. I think that fans of Twilight the book and the movie will be surprised by the Jacob Black that Taylor will bring to the screen in New Moon; and I’m looking forward to working with him and the rest of the cast in realizing the film.

very best

Chris Weitz”

I’d just like to add that I was very much a part of this decision. My first priority was always what was best for New Moon—what was going to give us the best possible movie. I’m truly thrilled that Taylor was the one who proved to the director, to Summit, and to me that he is the best possible Jacob we could have. And I’m very much looking forward to seeing what he’s going to bring to Jacob’s character this year.
- Steph