Last week during the GDC the Injustice:Gods Among Us mobile app was announced. It has currently been released on the App Store. With a healthy mix one part fighter, and one part collectible card game it should serve as a companion piece to the console version.
The mobile app will require you to set up a WB-ID that will allow for unlocks, and to share items between the mobile and console versions. Seems that the closer we get to the release for this game WB and NetherRealm really want to get our blood pumping.