Assassin's Creed: Revelations will launch a "Mediterranean Traveler Map Pack" on January 24, 2012, according to GameStop. The store's page has no further description of the DLC, except for a (potential placeholder) price of $9.99. Ubisoft has yet to confirm any details.
According to the AC wiki, the Mediterranean pack will include new multiplayer maps set in Siena, Jerusalem, Dyers, San Donato, Firenze and the Imperial District of Constantinople.
We'll likely find out more after AC:R's "Ancestors Character Pack" launches on December 13 for $3.99 (320 MSP). That pack will add four new characters to the multiplayer stabfest.