To me, here are the possibilities, in order of likelihood:
1. Punk retains, keeps title until Royal Rumble to meet Rock. What happens there? That's discussion for months from now. As for now, I see Punk going to Rumble to face Rock because Rock/Cena's been done, why do it again on a (slightly)smaller PPV ? Another split possibility is that Punk loses the title at Summerslam but retains it to meet Rock at RR.
2. Cena wins, keeps title until RR to meet Rock.
As I said in #1, why would WWE do Cena/Rock II on a (slightly) smaller stage? $$$. To boost Royal Rumble buyrates, they'll have the rematch at Rumble. I bet Rock will win the title at Rumble but lose it at Mania. Rock won't be on Raw every week, even if he wins the title at Rumble. The IWC, myself included, won't like this decision. I think I speak for a lot of people when I say that I'd love to see Rock/Punk. On the mic; those promo's would be gold. In the ring; Punk's one of, if not the, best wrestler in the world. Rock isn't awful, but he's not good. Punk would make the match legendary.
3. Big Show wins.
If Big Show wins, there's no way it'll be clean. WWE won't bury their two top talents for a guy that has as much time left as he does talent (not much in either sense.) I bet there'll be a run in, here, maybe even Rock, who knows. I just hope it won't be Big Show vs Rock at Rumble. Two guys that can't wrestle, and need I say more? I like wrestling matches, not the old 80's style Hulk Hogan matches, where the face gets destroyed by a bigger man, then suddenly no sells the entire match and hits a comeback and wins.