In the context of a touch screen, it's unclear that either QWERTY or Dvorak is the right answer. Dvorak was optimized for eight fingers going all at once, which isn't at all what you do on a touchscreen. QWERTY was optimized to maximize the likelihood that the type-bars on old typewriters wouldn't collide (so you want a lot of horizontal distance between adjacent key presses). When you think about trying to disambiguate a word being slammed out with lots of errors in it, the QWERTY layout isn't a bad place to begin, because anything that would have lots of near-adjacent key hits would be more likely to be ambiguous.

Consider, for example, the way that Dvorak puts all the vowels together on the home row. That's fine when you're on a keyboard but it's disastrous if you're fat-fingering a smartphone, since there are lots of words with consonants in the same position but different vowels.

I suppose the jury is still out on the absolute optimal way to do text input on a touch screen. I love SwiftKey. Others love Swype.