A subtle clue in a newly released Street Fighter X Tekken screenshot reveals two more playable characters for the game: M. Bison from the Street Fighter series, and Ling Xiaoyu from the Tekken series. Though neither character is seen in combat, they're clearly selected as the tag team alternates.
The screen from the Xbox Live Marketplace (via Shoryuken) actually shows Heihachi and Dhalsim throwing down, and the two character portraits are relatively small and obscured. This reveal is either accidental, or Capcom is testing how closely we're paying attention.
Both characters are mainstays in their respective series, so not entirely surprising, but they play very differently. Bison executes powerful maneuvers, while Xiaoyu focuses more on speed. This brings the roster up to 33 characters -- 15 from each game series, along with a few other guests.