Rock/Cena may or may not be for the title, but I don't think the Rumble winner is going to be involved in that match regardless. I think it will be for the title though, because I do not think Vince wants a repeat of Rock/Hogan where that match completely overshadowed HHH/Jericho and the crowd was dead for the title match.
I happen to think Punk is going to win the Rumble, and I think he is going to use his main event slot to either demand a match with Austin, or if he can't go, the Undertaker. I would be shocked if a big time Mania match wasn't part of Punk's new contract, and those are the only 2 opponents (unless HHH gets back in the ring) to justify the slot