It won't happen.
Smartphone for bitcoin mining? you are just destroying your phone.
Smartphone as digital wallet, you can simply download an app / wallet and that's how you make it as your wallet.
You never know! With the help of nano technology we may be able to achieve smaller chips that produces as much power as a full sized mining rigs in future. I mean that is not the reality now but it may become in future. If you were 80's kid, did you ever think that you can carry a computer in your pocket? Because what computer we have seen that time was Pentium one and it required an air conditioned room to run that. Nano technology achieved that impossible and now we carry a smart phone inside our pocket.
So keep your hope high as we don't know where the technological achievements can take us!
It may happen since we are already in revolutionary era and developing with that will become true someday. If we will back to the year 19's, all of this thing we experiencing right now is almost an imagination to them but look how does the revolution process give us an advance technology, we can easily innovate things by the help of science and other field. Therefore, it is not impossible.