I think the rumored plan for him being at Extreme Rules wasn't Cena/Rock III, but rather Cena/Rock vs The Shield, with The Shield going over. That would have been all kinds of awesome, unfortunate it didn't happen. Much better than what we got now with Ryback.
I also believe he's wrestling one last time against someone (Brock Lesnar) at Wrestlemania XXX. At this point, I wouldn't put it past WWE to have Rock purposefully feed people these comments about "my last match was almost certainly at WM29" just so people will be more shocked when he comes back again next year.