Here's the trailer. It's pretty awesome, though it doesn't give much of a plot or info about the game.
Also, they released a bunch of screenshots and a bit of art, seen below:
In terms of plot, they released a little bit, but not enough to spoil anything.
We're back with good ol' Ezio Auditore, running around Constantinople kicking some templars in the nuts. You'll get a bunch of new weapons and abilities, including the hookblade, as you can see in the first screenshot. You'll also be able to craft bombs and you'll have better eagle vision. There are also a couple of cool new locations like the Assassin's stronghold in the mountains. As I'm sure a lot of people hoped, Altaïr is also in the game as a playable character. There are of course a couple of other new NPC characters such as Prince Suleyman. And for even more good news, they said it would have a refined multiplayer with cool new game modes and whatnot.
In summary, this looks like it will be an awesome game.