This Street Fighter X Tekken reveal video shows the -- via PS3/Vita-exclusive DLC -- unlockable characters Mega Man and Pac-Man. There are no such playable character unlocks on Xbox 360 during the launch month, but there will be others in the future.

This website lists all the unlockable characters and how to unlock them in Street Fighter X Tekken:
http://www.videogamesblogger.com/2012/03/06/how-to-unlock-all-street-fighter-x-tekken-characters.htm