Yeah pretty much. That's the reason that Cena vs. Rock at WrestleMania this year wasn't for the WWE Championship. The Rock isn't going to stay around and be a full time wrestler, or even a part time wrestler for that matter. Everybody knows that because his movie career has really picked up over the past year or so. In 2011, he made almost a total of $40 million for the movies he played in. Next year, he has six different films scheduled to be released including a G.I. Joe and the next film in The Fast & The Furious franchise. Having Cena face Rock at WM for the title would've given away the ending long before there was even a match.
However, things will be a bit different for next year's WM if Rock is able to compete. There's still a question as to whether or not he'll wrestle with such an intense schedule. After all, the new G.I. Joe movie is slated to open one week before WM so Rock will have to devote a ton of time and energy into promoting the film, giving interviews, going to premieres, etc. If he is able to compete, however, and it does turn out to be a Cena vs. Rock rematch for the WWE Championship, the WWE isn't counting on unpredictability. If Cena & Rock wrestle each other again, it's a pretty safe bet that John Cena will win. I think a lot of people think that as well. I just don't see John Cena losing to Rock in two straight WrestleMania matches, especially if this hypothetical second match is for the WWE Championship.
If the match does take place, I just don't think it'll have nearly as big of a feel about it because people will be expecting Cena to win the match in order to even things up. And, as has already been pointed out by other posters, nobody is going to expect Rock to walk out of WrestleMania as WWE Champion. WWE could get by with having Rock carry the title for a month or so after the Royal Rumble but Rock will be gone again after WrestleMania and everyone knows it. So yeah, if this match goes down, the WWE will probably build the match as John Cena's search for redemption after losing the biggest match of his career to The rock at WM 28.
At the same time, however, as I said, there's a question as to whether Rock will have the time to wrestle at WM. There's also John Cena's current injury issues to consider. As of right now, there's no real solid clue as to whether John Cena will compete against Punk at HIAC. There's talk that he'll wrestle HIAC before taking some extended time off to heal up. Aside from recovering from his elbow surgery, Cena's pretty banged up and has a lot of issues going on with his neck, his back, his hips, etc. In comparison, Cena's elbow issues are pretty minor. What I'd personally love to see happen would be for CM Punk to go into Royal Rumble and beat Rock as it goes right along with his search for "respect" and to further solidify himself as the "best in the world" before ultimately facing John Cena at WM in some sort of huge blow off match to their feud. I don't think that'll happen, however, as I'm sure Vince has his heart set on Cena vs. Rock II. I'd just personally love to see Punk get a win over The Rock.