Speculation was rampant that the start of production was imminent, though there wasn't any further evidence pointing to that. Director Michael Bay said they couldn't start shooting until at least May 2008, which got me to thinking about what actually may be going on.
I think they will shoot various scenes, off and on, between now and the actual start of production, so the FX guys (I'm guessing ILM again) can get to work on various components. Heck, they shot the train sequence in Spider-Man 2 at least 4-6 months before the start of production, before the final script was done.
Onto other rumors: IESB.net is claiming to have a casting breakdown of the movie, and that the Constructicons (and Devastator) will be in the movie. Three problems with that: a couple of Decepticons in the first film had Constructicon names (Devastator/Brawl, Bonecrusher), Bay has said they won't make it into the sequel (he said the same thing about Dinobots--ME GRIMLOCK NO LIKE THAT!), and Bay also likes to tease us fans and movie news sites with false information.
On the plus side, misinformation could actually mean the Dinobots and Constructicons will indeed be in the film. Plus, we know that both Starscream and Barricade survived the last movie. Come to think of it, Barricade was last seen running point for Bonecrusher when he transformed on the highway, and that was it.