The Mechromancer will be an all-new playable character that Gearbox plans to develop after work on Borderlands 2 is finished and the game is in certification.Basically it's a DLC annoucement... "Currently, she exists as an idea and a couple of sketches, but we expect her to become a reality at some time after
Borderlands 2 is completed." E.G. although it might seem that we're working on this DLC now, we are not.
"As with many new ideas, we don't yet know exactly how the Mechromancer will play or when she will be available, but our estimate is that she will be released within 60 to 90 days after the launch of
Borderlands 2. At that time, we also expect to release a compatibility update so that people who have the Mechromancer can play with those who don't." E.G. yup, she's DLC alright.
Gearbox says so...
here.