I honestly haven't heard some people still say that he'll turn heel before Mania, and I wouldn't listen to them if I had.
If Cena were going to turn heel, let him have enough time to put together a run where he tears through all the other babyfaces. Don't change his wrestling style, let him remain as Super-Cena, and then let someone beat him at Mania for the strap, probably the guy who was going to be the next huge babyface.
Going by my notion of how I feel it should happen, I'm just going to say that I don't think it'll happen anytime soon because I don't see anyone on the horizon that's going to take his spot with the Make-a-Wish Foundation and all the red tape that goes with being the top guy. I don't think Orton has it in him to be that type of guy, and a guy like Morrison is still a ways away from that. I probably think that by the time something like that happens, Cena will be ready to retire from wrestling and there will be a new cast of babyfaces ready to move in. Like I said, maybe Morrison could be that guy, but I don't know. It'll probably be someone who's in FCW or not even in the company, right now. Also, I don't think I could ever see Miz being a babyface. He's got that smugness about him that I don't think kids will ever get behind, and he's got the backstory of a heel; he's the character that isn't going to beat you honestly, because he can't. Not saying Mike Mizanin is a bad wrestler, it's just his character isn't the type that will ever out-brawl you or out-wrestle you, and I don't think he'll ever be booked differently. So, no, I don't think the Miz could ever be a successful face, at least not in a children's market like the PG era is, and he won't be the guy to take Cena down if Cena turns heel.
Finally...I just have to admit that at this point, I don't think that I ever want Cena to turn heel. Sure, Cena is kind of boring and at some point, if he wants a really lengthy career as a wrestler, he'll have to turn heel to stay relevant, but I just feel that his character is forever fated to be a babyface. Now, I do realize that the Hogan similarities are impossible to ignore, so I admit that he might turn heel like Hogan did...again, once he's older and has to turn heel to stay relevant...just like Hogan.
In summation, no, Cena will not turn heel anytime in the near future, and no, Miz will not turn face. I'm sure the WWE knows that they'll always have a Cena heel-turn in the works if they ever need it, but they aren't going to need it probably for at least another year or two.