The kid in me has just gone insane with glee. It's proper name is 'Street Fighter X Tekken' and it#s gonna be done in the style of Street Fighter 4 BUT Namco's also working on 'Tekken X Street Fighter' Which will be done in the style of Tekken 6.
Here's the first trailer for the Street Fighter version.
Also, here's some Screenshots.
Nothing much has really been revealed of the Tekken version of the game though. Both versions will be released some time next year, the only hint they gave is that it'll be after Marvel Vs Capcom 3.