The Coraline team are constructing puppets and costumes and armatures and they are building sets and making objects -- any single thing you see on the screen has to be made, after all.
They are also doing technical tests -- it's easy enough for me to say in the book, and for Henry to put into his script, that as Coraline walks away from the Other House, the trees are less like trees and more like the idea of trees, but making an orchard turn into a misty abstraction is easier said than done when you have to build it. So they've built one, and are doing their camera tests to see if it will work.
They have walls and walls of production art, posters, designs and so on.
I'm not sure that the difference between pre-production and production is as clear-cut on something animated as it is in a live action movie. March is when the camera starts rolling on the content. But they're certainly making their movie right now.
(Incidentally, my favourite tiny thing about the Laika Studio -- a 151,000 foot warehouse -- was noticing that the "Mission Statements" painted on the walls by the previous occupants have been elegantly graffitied over, with splashes of green paint spray-painted over with big black buttons. )
Incidentally, on the subject of stuff leaking out etc., it is with a certain wry amusement that I notice that the mission impossible team over at Ain't It Cool News has a bunch of art from Beowulf up. It's art taken from the Art of Beowulf book, to which I am currently writing the introduction. And my wry amusement derives from the fact that all I've been permitted to have at home of the material is teeeeeeny tiny images on photocopies, and now, online at aint it cool, they have the real thing and up digitally.
(Probably worth pointing out that this is "concept art" though. Some of it is simply a "what if we try this?", and not the way something will look in the finished film.)
So now I'm hoping that I can get some Coraline stuff up here at least before they put it up on aint it cool news...
[Edit to add: the pictures have now vanished from Aint It Cool, I'm afraid.]