This just got announced seconds ago!!! Street Fighter vs Tekken is indeed real and a trailer was just shown at the SDCC Sf panel! We will be uploading the trailer as soon as it becomes available.
The trailer shows dan smashed against a wall and Ryu and Kazuya Mishima fighting each other in an ink style trailer. Street Fighter X Tekken will be coming to PS3 and Xbox 360 next year.
Ono then surprised everyone by showing a demo of the game. The game plays like SF4, which mean strictly 2d gameplay with 3d models.
UPDATE: this is just in!!! Harada san, tekken producer, announced that another game titled Tekken X street fighter (namco style) is also in developpement