This is based on a YA novel
of the same name by Philip Reeve. In my opinion, it's one of the very best YA science fiction novels out there. In fact, I would put it equal to Harry Potter in character development, plot, and humor, and above Harry Potter is moral complexity and intelligence. I was very excited to learn that it would become a major movie produced by Peter Jackson, and directed by Christopher Rivers, who apparently did storyboarding and special effects with Jackson in the Lord of the Rings
So first thoughts: great trailer. The biggest danger with a steampunk-type movie is that they'll go overboard trying to make everything dark and gritty, but in this trailer everything has bright colors and occurs in broad daylight. The idea of a huge city rolling across the landscape is hard to pull off but I'd say London looks pretty good. Glad to see that they have St. Paul's Cathedral mounted at the top of London, which is a detail mentioned in the book.
One thing that will be interesting to see is how they portray Hester Shaw. The book says that she's hideously ugly because of a huge scar across her face. Of course, Hollywood rarely approves big-budget movies with an ugly woman in the leading role. Here they've solved the problem by having a scarf over her face, but one assumes they can't escape the dilemma for the entire movie that way.