This idea sucks on so, SO many levels! It's only doable on Android as a Live Wallpaper. As a messaging app which could run on Android and iOS it solves jack since you usually look down on the phone so the ability to see what's behind the phone doesn't add much, especially since the keyboard will obsure some of the view. The text needs to remain legible no matter what you have in the view so have fun creating an algorithm that determines whether you want black or white text (assuming you want the same behavior as iOS). Perhaps you want to add some blur to ensure that the text stands out.

And finally: The user will probably focus on the message so there's a chance that the user will crash into someone or something anyway.