According to Variety, the Inception co-star will play Alberto Falcone, aka The Holiday Killer. The son of crime lord Carmine Falcone (portrayed in Batman Begins by Tom Wilkinson) plays an important role in the comic mini-series The Long Halloween and its sequel Dark Victory, which were early inspirations for Christopher Nolan's re-imagining of the Batman universe. Given Nolan's intentions to keep his Gotham City grounded in reality (and Gordon-Levitt's resemblance to Alberto Falcone), working a character like The Holiday Killer into his universe makes a lot of sense.
Another unexpected announcement is that Juno Temple (Atonement, Kaboom) is up for an as-yet-unrevealed role as well. The report states that Temple is being looked at to play a "street-smart Gotham girl."
In addition to Gordon-Levitt, The Dark Knight Rises will star Christian Bale, Tom Hardy, Gary Oldman, Morgan Freeman, and Michael Caine , and is still scheduled to swing into theaters on July 20, 2012.