How about a happy medium: Cena cuts a backstage promo near the beginning of the show and Miz attacks him. That way, Miz makes a huge impact and he doesn't get in the way of the match. You want to even the odds? Have somebody attack Rock too, but don't reveal who it was. Then Rock has an opponent for Summerslam (and a reason to fight them!!!) after he puts Cena over. Ideally it could be a spot for an Orton heel turn or a brief Punk turn which would work perfectly with his summer 2011 promos.
Or instead, Cena comes out for the match injured, as a true underdog, in a last-ditch chance to gain some favor with the crowd.
Edit to add: I do realize it's a stretch to use Wrestlemania to set up future angles when WM should be an endpoint. But if they're gonna have this Rock/Cena exhibition match with no stakes, I'd rather get something out of it going forward.