From 1UP.com: “Animal Crossing faithful, Nintendo has not forsaken you. To help celebrate the launch of The Legend of Zelda: Twilight Princess, the company will be distributing an exclusive item in Animal Crossing: Wild World starting on November 17th and running through November 24th.
In order to receive the letter that will unlock the treat, you have to log into Nintendo Wi-Fi between the aforementioned dates. Log in and report back, Infendo. My guess? Nintendo finally introduces the Master Sword to AC and ramps up the violence to Gears of War levels.