The demo is out on Tuesday 14th February.
Early access is also possible via this Facebook game...
http://t.co/gjx6RHfI
I've just finished playing the demo. Love it. It features two sections: one from early on in the game, on Earth, and another from later on (the same later section demoed at the Eurogamer Expo last September).
There's also multiplayer, but that's not playable until the demo goes public on Tuesday.