I have the worst story about this. I had just graduated college, but I immediately found a job. My friend's dad, who I owed more favors to than I could count (he was like a second dad to me), asked me if I could help his friend with an app idea he had. I told him I would be starting a new job soon, and I doubt I would have time to commit to that. He asked me to at least hear this guy out, and I figured, what's the harm in just listening to this guy's idea? I could always just say no.
So he set it all up. His friend would call me and we would talk about his app idea. So the day came, and the guy called me, and I realized quickly that this guy was treating this like a job interview, not like, "I have this idea, can you help me?" But he wouldn't even tell me what the app idea was. He kept saying things like we would need to sign an NDA and it was a trade secret. He was asking things like, "what could do for me? What could you bring to the table? Why should I consider you for this position? Why are you the right person for the job?" Finally, I had enough, because I didn't even want to talk to him in the first place, let alone help him with his app, and now he was acting like he was doing a favor to me. So I said that I'm not the right person for the job and that I think he should find someone else.
He immediately changed his tune and started trying to tell me about his app idea to keep me interested. His app idea was to essentially have what the camera is seeing be the background for your phone, so you can use your phone while driving and still see what is in front of you. He acted like it was some sort of great safety app to protect pedestrians and bicyclists. I told him flat out that that was the worst idea for an app ever and that it's almost certainly illegal. And that was pretty much the end of the conversation.
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.
There was an SMS app for android that did the whole camera view as background so you could text while walking. I never tried it because it seemed like a bad idea.