The Star Wars: The Old Republic site has been updated to let the community know that the next major content update is launching in December. Jedi Under Siege takes place on "the ancient Jedi world of Ossus" where the Sith have invaded. Players will be able to support either side of the struggle irrespective of their current player faction. "Will you support your own faction, or secretly sabotage their efforts in order to support your former enemies?"
Jedi Under Siege includes a number of new features including a new storyline, a new planet to explore, new NPCs to meet and "team up with beloved Companions". In addition, there will be extra daily missions, a Master Mode Operation, and new gear to collect.
You can check out the details on the Star Wars: The Old Republic site.