k-essential phantom energy: doomsday around the corner?

In spite of its rather weird properties which include violation of the dominant-energy condition, the requirement of superluminal sound speed and increasing vacuum-energy density, phantom energy has recently attracted a lot of scientific and popular interests. In this Letter it is shown that in the framework of a general k-essence model, vacuum phantom energy leads to a cosmological scenario having negative sound speed and a big rip singularity, where the field potential also blows up, which might occur at an almost arbitrarily near time in the future that can still be comfortably accommodated within current observational constraints.