It's official, Borderlands 2 is in development and coming out sometime after April 2012.
Game Informer's running it as a feature of their September issue:
I had so much fun with the first one, can't wait to see where Gearbox takes it next. Not long to wait as they'll be showing it off at Gamescom in a couple of weeksThe September issue of Game Informer contains 12 pages of exclusive details and screenshots from Borderlands 2, and outlines the numerous improvements the developer has in store for your return trip to Pandora. Don’t expect just a couple bazillion more guns this time around. Gearbox is introducing new and more diverse environments, smarter and more powerful enemies, and an improved mission system capable of delivering a better story. This and much more will be revealed in the story, as well as our full month of additional online content.