Borderlands 2 just got announced and is being developed by Gearbox Software.
Release date is set to 2012 and will feature all new characters, areas, enemies, equipment and weapons.
More info can be found at Gamescom and Pax Prime this year where the game will be shown, Game Informer also has the exclusive article where more info has been shared about the game.
http://www.borderlands2.com