Street Fighter x Tekken Mobile offers "a wholly new experience" based on the console version. It will be compatible with iOS 5 or higher, meaning you'll need an iPhone 4 or iPad 2 or better.
According to Capcom-Unity, the best way of describing the experience is "SFIV Volt, but with tag mechanics and a healthy dose of juggling." Apparently, tagging in the second character will function differently, but we'll have to wait to find out more about that.
Capcom hasn't announced characters or modes--but presumably, the company will manage to avoid the "DLC on disc" controversy that the console originals had to face.