Borderlands 3 is already in the works according to the Gearbox Software's lead writer Anthony Burch. Let's not get ahead of ourselves though, the game is not in production and it has not been officially announced. That being said, it's reported Burch mentioned on Revision3's The Totally Rad Show, he and the rest of the Borderlands team were talking about their ideas for Borderlands 3.
Nothing major to report since Borderlands 2 has been such a hit, but it's nice to know they're planning for the future. We've talked to Randy Pitchford enough times to know Borderlands 2 will have plenty of DLC support, so if we do see a new Borderlands game, it won't be for a while.
Source: VG247 via MP1st