AusGamers has a video interview up with Assassin's Creed 2 associate producer, Vincent Pontbriand, that goes into a fair bit of information about the game including using gondolas to travel between cities, how Desmond Miles will fit into this game, the interaction you'll have with the future (present) and its newfound structure with the past as well as the new dynamic AI system and much, much more.
www.ausgamers.com also have a full transcript if you can't see the video.
From the interview:
"Vincent: The separation between past and present will be more blended, if you will. Instead of a few moments where you visit the present, you'll have fewer of those but they're going to be more action-packed. And the present is going to be more there with you as you're doing your past missions; you'll hear people from the present monitoring the Animus and giving you hints when you need them."