IGN has an interesting look at Assassin's Creed, including some informative notes on how you actually control a character as fluid as the assassin Altair:

In order to control Altair, Ubisoft attached four "puppet strings." One to his head (the Y button) his weapon hand (X), off hand (B) and his lower body (A). The right trigger has been jokingly dubbed "emotional control" by Raymond and her team. In short, it is a strength modifier. While walking, hold it down to run. While swashbuckling, hold it down for a lumbering power strike.

Using Y, you will do things like eavesdrop and use intuition. A, then, would be used to jump. An interesting control feature is climbing, done in a way we've never seen before. We didn't get a chance to touch the controller, but you will actually feel for handholds, like rock climbing.

I wonder how feeling for handholds will be handled.My guess would be the best way to do it would be rumble, but that would only be viable on the 360. And speaking of the 360, apparently the crowds will be smarter in the 360 version, thanks to threading. Of course, the concept of playing a game where primary gameplay is trying to push your way through crowds of too-slow assholes doesn't quite appeal to me, no matter how smart they are. That's already what I deal with walking down Grafton Street every day. Florian Eckhardt

Assassin's Creed Preview [IGN]