I noticed over at reddit there was a big fuss over what was initially thought to be a trailer for the game but is actually (or so I'm led to believe) the opening cinematic to the game - as in the first thing you see when you click on 'campaign'.
I was gonna post it in the Halo 4 official thread but I went back a few pages and all the guys there wanted things keeping spoiler-free, so I thought it was worth making a new thread, appropriately marked so all of them could avoid it if they wanted! Anyway, without further ado:
http://minus.com/lG2SvytaHHbsL
Not sure how long it will keep up since it's a mirror of a youtube video which has long since been taken down, but enjoy while it lasts!