Don't watch this trailer if you didn't finish Assassin's Creed Brotherhood. If you did, this is a tease of what happens with series hero Desmond in this fall's Assassin's Creed: Revelations. Looks to me like the Desmond parts of this game will be like the virtual-reality training missions of Brotherhood. But that's just a hunch.
We'll have a lot more Revelations impressions soon.