Well hold your horses there, friends... I don't think Rock will be coming back on a full-time basis as some of you are thinking. Now all he did was not rule out another match, and of course given the current promo-feud between him and Cena it is more than likely to come to a Cena/Rock showdown somewhere down the road - and it should be awesome.
Now me personally, I'd actually prefer it if they did Rock/Cena at Summerslam instead of WM28. Why? Simply because you could save Rock/Taker for Taker's last Wrestlemania match in that case. Besides, keeping the feud between Rock & Cena running from now until WM next year will be TOUGH. I mean, WWE are already having problems right now running the current WM feuds into WM 27, merely because they have to cover 6 weeks of programming for these particular feuds, instead of the regular 3-4 weeks they usually have between PPVs these days. So how will they keep the thing fired for over a year? Seems quite tough to me...
So I'd say, Summerslam should be the place to go for Cena vs Rock, and then aim for a big Rock/Taker showdown at WM 28. It would make the most sense: The Rock wouldn't be hurt by losing to Taker at WM since he is no WWE mainstay anymore, and it would be a bigger match even than Cena vs Taker, which of course would also be quite huge. But the idea I could see behind it is this: Rock has been going on in about every promo since his last run in WWE seven years ago, that "he's done it all in this business" and that he achieved everything he wanted to achieve. Well, WWE could take that ball, and have Rock come out and say something like: "Well, The Rock has been thinking, for the past seven years, that he's done it all in this business. And he has. But then, after The Rock left for Hollywood, year after year, there was one thing that suddenly became bigger and bigger... and now... well now it seems there IS something left that The Rock hasn't done yet. Because no one has done it yet. And that is... beat The Undertaker at Wrestlemania." And there you have your epic showdown match setup, and probably the highest note possible you could send Taker riding out into the sunset. Cena/Taker would be big of course, but Taker should win, and a loss would... well not hurt Cena THAT much, but still wouldn't help him. Rock on the other hand can afford to take a loss to Taker at WM, and Rock/Taker would be even a bigger match than Cena/Taker; simply because it would be two absolute legends going at it, whereas Cena, as big as he may be today, still has a way to go to reach "legendary" status.
So yeah, that would be my ideal plan... Rock/Cena at Summerslam, and Rock/Taker for WM28... and Rock really doesn't need to fight ANY more matches than those two if you ask me, haha.