Find it hard to fathom that a Rock / Cena rematch will happen, at any point. While Vince and the WWE aren't exactly big on standing storylines, I really think the "Once In A Lifetime" angle they ran at WrestleMania between The Rock and Cena will hold up longer than 3 1/2 months, through SummerSlam and even past WrestleMania next year.
Cena's "announcement" will likely have something to do with next week's 1000th Raw, but I don't think it will have anything to do with the MITB briefcase. Keep in mind, Vince and the creative team like to use Cena to make an impact, not necessarily in a match sort of way or in a shocker sort of way. But he's the company ambassador and the "announcement" may just be a general storyline or surprise we'll get next Monday night.
Then again, dude might announce he's cashing in on Punk or Sheamus next Monday, at SummerSlam or even tonight. We'll see shortly.