Hudson also announced that no new squad mates would be added in the DLC, nor would there be in any future DLC. We'll be sure to update as soon as EA makes a formal announcement with more details.
Update: The official Mass Effect 2 website has posted a more detailed description of the scenario (reproduced after the break), along with the price: 560 MS/BioWare points ($7).
"When sent to investigate a Cerberus research base that's mysteriously gone silent, Shepard arrives to find Geth overrunning the base. The sole survivor, Chief Scientist Archer, paints a dire picture: an experiment to fuse a human volunteer with a virtual intelligence created a dangerous hybrid "VI overlord". The rampaging VI has already attacked three other Cerberus bases, controlling any technology it finds in an attempt to break free–and unleashing Geth across the planet. Unless Shepard can infiltrate the VI's fortress and shut it down, this homicidal intelligence will beam itself-off planet and wreak havoc on other systems. The action unfolds over five new level areas, with two new achievements."