BioWare has updated the Mass Effect 2 site with information on Kasumi, an "enigmatic thief that has "enemies everywhere".
Part of the upcoming DLC Kasumi’s Stolen Memory, she is the newest and final addition to your team.
Apparently, not much is known about her, other than "Cerberus has contracted her to assist Commander Shepard in the mission to save mankind" and in order to get her help, you must first assist her in infiltrating the vault of criminal art collector Donovan Hook.
Sounds like a plan. Thankfully, gamers can expect more bits like this too keep coming, as Bioware told us at GDC that even more content is on the way for ME2, and there should be enough to keep you occupied for a while yet.