It makes sense that he'd be announced now as the new CW show Arrow debuts tomorrow night.
Injustice will be available in April 2013 for Xbox 360, Playstation 3 and Wii U. We'll catch up with Warner Bros. Interactive this week at New York Comic Con to see if we can dig up any further information.
There is a non-embeddable video over at EW.com. Head over there to check out the gameplay. We'll post it here onsite when it becomes available.