The user you replied to said "Not really an easter egg as everyone that plays the game sees it..." but he was actually referring to his own Easter Egg, not the Assassins Creed 2 easter you mentioned before.
[spoiler:7a5a56d50f]You meet a guy named Crazy Joe near the swamps in the Miracle Cure quest. He is located in a small shack near the swamp. Take a close look at his T-Shirt: