Altair's name in Arabic means "Flying one" or "Bird of Prey", his last name ibn La-Ahad means "Son of No-one".
Altair is missing a finger on his left hand; unlike the rest of his order. Theories are that he had the finger removed to accommodate a blade of his own design.
Originally, the game was to include a crossbow for ranged attacks but Ubisoft had this removed to make the game more challenging (as test gamers favored the ranged attacks over mêlée combat). Although the crossbow doesn't feature anywhere in the game play anymore, the evidence of it's existence is still in the trailer at the start up of the game - Altair is wearing it on his back along with a quiver of bolts. It was also removed due to historical inaccuracy.
As a joke, the game designers hid the image of Optimus Prime (Transformers) on certain buildings as an Easter Egg. The image appears as two boarded up windows above a triple shuttered window and can be spotted in more than one city on a number of buildings.
At the beginning of the game, Altair is 26 years old, the same age as Desmond Miles.
Was originally envisioned as a Prince of Persia spinoff called Prince of Persia: Assassin where you played as the Prince's bodyguard when he was a child. UbiSoft though didn't like the idea of a Prince of Persia game where you didn't play as the Prince so the game was turned into a new IP instead.
Prior to doing the music for Assassin's Creed, composer Jesper Kyd worked on Eidos Software's Hitman video game series, a game featuring an assassin as the main character.