Ubisoft shed some more light on Assassin's Creed with a demonstration, where plenty of tidbits could be gleaned. The level demonstrated was Jerusalem, using ancient texts as a source for creating the environment. Speaking of the locale, every building visible in the city can be climbed by you, and all are accessible, which definitely is good news.
The combat system is allegedly advanced, where your performance is dependent on well timed button combos, and countering your opponent's attacks, based on their animation. When hunting down a target, you may end up chasing them through the city streets. The "Chase AI" is advanced and will make each chase a different experience, especially since your target's mannerisms are dependent on their personality.
Also, through the demonstration, characters and the immediate area around your character would see flickering and feedback, giving it a psuedo "Matrix Code" feel. All in all, Assassin's Creed is looking like a solid winter release, though we'll have to see how it compares to the big guns.