I would hope that the titles would be unified (on Raw and SD! anyway), but I am not sure how likely that possibility would be.
If I had to guess now, I would suspect Randy Orton will leave Wrestlemania 24 with the WWE Championship, Mr Kennedy will walk out with the World Heavyweight Championship, after cashing in his MITB opportunity, turning face in the process. As for the ECW World Title, I would suspect CM Punk will be that Champion, although you can never tell with that brand, as another green big man could enter the frame and get a title run.