^^ Sorry to say, but you're all wrong.
They couldn't have possibly started shooting New Moon. The reason being, a movie sequel cannot be filmed before the release of its predecessers because the production house cannot predict whether or not the first film will make enough money, past it's film budget to warrent a suggestion for a sequel to be made.
The reason sequels are made is because the first one was sucessful. New Moon movie will be made ONLY if Twilight is successful and becomes an established "bankable franchise" (read money-making horse). If Twilight fails at Box Office, the sequels will not made. Alternatively, if Twilight does might well at the box office, New Moon will be made.
It is true that the actors have signed on for sequels but it does not inherently mean that there WILL be sequals. Scripts already written wil be cancelled, contracts will be vetoed and so on if the first film fails at Box Office. It's all about the money. The sequels of Harry Potter weren't a sure thing either. It's only after the first one made considerable money that they started working on the next and so on.
Not saying there shouldn't be a movie, just saying that people should be a bit more realistic ;) This is how Hollywood works.
P.S. They do NOT need to make Breaking Dawn into two movies. Harry Potter and the Deathly Hallows has enough plot for two movies. Breaking Dawn doesn't. :P