BioWare have released a cinematic trailer for Mass Effect 3, showing the beginning of the War for Earth..
[youtube http://www.youtube.com/watch?v=eBktyyaV9LY]
The demo is available now, on Xbox 360, PlayStation 3 and PC, with both singleplayer and multiplayer sections. The full game will be available on the 9th March.
Our impressions of the single-player demo can be found here.