Sorry to say, but the E3 trailer does not exict on XBL at this moment.
Yeah. I know. It sucks. I've made it a habit to check almost everyday on LIVE. The PSN got it the day that it was revealed at E3 but I use my 360 alot more. I like to have a collection of trailers of games that I like to remind me how awesome they were so
that I play them again. I have all the trailers of the previous Assassin's Creed games on my Xbox 360, I just need this one.