Gearbox Software has already mentioned that they are eager to do a follow up to the successful Borderlands if an opportunity was given. And according to VG247 they have now registered the trademark Borderworlds, perhaps an indication that we won't return to the desert planet Pandora that featured in Borderlands, but that we will get to visit more planets in a sequel.
Meanwhile, Gearbox have plans to do further updates of Borderlands with downloadable content. And there is no official word on the sequel yet.