The PDLC will make available a new thief named, obviously, Kasumi. Kasumi will be found on the Citadel and provide you with some new missions and tack on a new achievement upon their completion. The only details available about those missions are that they involve some stolen art and you will (assumingly) help in their recovery. During those missions you will unlock a new weapon called the Locust Weapon, with no details yet released about it.
Price and date of its release have not been set. "
Link- http://www.trueachievements.com/n1159/mass-effect-2s-first-dlc-to-purchase.htm
EDIT-"The DLC, according to Joystiq, will be the first bit of paid for DLC outside of the Cerberus Network and will offer around an hour and a half of gameplay, a "Locust" SMG and a flashbang grenade loyalty power to learn. Oh, and one achievement.
The short episode will revolve around Kasumi, a human thief who you can bump into on the Citadel and recruit into Shepard's squad. Her loyalty mission involves a secretive recovery operation that will see Shepard and co. have a run in with an influential art collector.
Kasumi's Stolen Memory is scheduled for an April 6th release for a price yet to be confirmed. Of course, before that we'll have the chance to sink our teeth into the free Hammerhead DLC missions which we should see before the end of the March."