There have been rumors swirling about Boarderlands 2 for a few days now and 2K Games just said fuck it! They announced it earlier this morning officially. They also teased there will be new characters, skills, equipment, and weapons scattered through out Pandora.
They also gave out a vague release date window. Sometime after April 1, 2012 during the companies next fiscal year.
And one other thing. If you want to get a glimpse of the game, you don't have to wait too long. They will have a booth both at Gamescom and PAX Prime. Gamescom is this August 17-21 and PAX Prime will be August 26-28.
source: 2K Games