I tried to keep this trailer as spoiler free as I could, but i would advise that you shouldn't watch it unless you have beat the game, or almost beat it.
Link: http://www.youtube.com/watch?v=pFpwfqtpuxM&feature=channel_video_title
Hope you enjoy, please tell me what you think below. If you don't like it constructive criticism would be appreciated.
There is also a similar thread on the steam forums.