If you were in any doubt that 2K would sanction a sequel to Borderlands, perhaps this will quell your fears: An indiscrete resumé listing suggests the game's already in the works.
Superannuation turned up a reference to the game on a freelancer's CV.
The developer apparently worked on elements of the un-revealed game's user interface.
Borderlands released in late 2009, and has racked up over three million sales so far. It was launched two years after reveal, suggesting any announcement in the near future would be at least twelve months in advance of release.
Creator Gearbox Software has been hard at work polishing off Duke Nukem Forever, and is rumoured to be involved in one other major project in addition to a possible Borderlands sequel.