It seems that all the previously announced characters in Street Fighter x Tekken came to fight a mad man, but instead they found a God. Cheesy Street Fighter The Movie quotes aside, Capcom has announced M. Bison as one of the final characters in the game. The other three are Akuma (Street Fighter) and Jin and Ogre (Tekken).
Also announced is the date for the PC version of the game, which is due on May 11 in the US.
Source: Press Release