The pack introduces three new maps to the multiplayer side of Connor's Colonial climber, including Charlestown, Fort St-Mathieu, and Saint Pierre. Three new multiplayer characters are bundled in too, namely the Governor, Highlander, and Coyote Man (presumably one of the less popular bartenders at Coyote Ugly).
Update: Ubisoft UK tells us the Battle Hardened pack is coming to PSN there on January 9, and to Windows PC on January 15. It'll cost £7.99 on PC and PS3, twice as much as the previous DLC, meaning the US price should be $9.99. Since the PlayStation Store updates on Wednesdays in Europe and on Tuesdays in North America, that indicates the DLC should be available on NA's PSN later today.