F4WOnline.com is reporting that as of now, WWE does not plan for The Rock vs John Cena match at WrestleMania 28 to be for the WWE Title.
While creative has gone back and forth as to whether or not the bout should be for the title, and John Cena did cut a promo after last year's WrestleMania claiming that he would be champion by next year, and would defend the title against The Rock, the feeling internally is that the bout does not need to be a title match.
Additionally, WWE management feels the company can get more mileage out of WrestleMania 28 by promoting a separate WWE Title match on the card, and leaving The Rock vs Cena as a non-title bout.