In my case I found it under the "Updates" tab in Gmail, only after I read this thread and went looking for it. It might be worth checking it out if you haven`t already.
Windows Phone devices already worked like that before Cortana. But in a more pre-scripted fashion. Something like
-- "You've got a new message from Paul, read it or ignore?"
-- Read it.
And so on (including the dictation and sending of the reply)
XAML and its Adaptive Design concept allows the layout to rearrange itself to better fit smaller or larger screens. And the programmer can always explicitly use different UIs depending on how many pixels are available to the app.
And everything I wrote is already happening (actually, it's been happening for a while). Just try any well-built W8 app in different screen sizes or snapping configurations. ;)