This screenshot, posted on Street Fighter x Tekken's Xbox Marketplace website, accidentally (?) reveals two new characters for the crossover title.
You won't see them punching; you'll find them in the character section up the top of the screen. They're Tekken's Ling Xiaoyu and Street Fighter's M. Bison.
For Tekken, the day Bison graced this game was the most important day of its life. But for Bison...it was Tuesday.