David; I think Sora didn't get it because..Ventus is asleep and has been basically been unconscious for years b/c he got badly, badly hurt and needed Sora's heart as life support. Otherwise Ventus's presence would be really obvious (Memory flashes, etc.). Remember when he had Kairi's heart? And even then he didn't get it until it was out and out pointed out to him and Kairi yelled at him from within in KH1.
If Ventus wakes up, Sora'll really know something is off, and I hope he'll find a way to talk to him a bit or get help. I'm guessing he long forgot that he let Ventus into his heart btw. One don't remember much when they're four years old after all.
But considering from the spoilers I read that Ventus actually did something in DDD, I think he's close to waking up; The Xenohort crew did try their best to shake up Ventus's heart after all to distract Sora, or that's the way I understand it. Which means if he wakes up first, I hope Sora can come to a understanding so they can coexist long enough for Sora to get Ventus back to his body.
I do think Mickey or Yen Sid'll bluntly point out that he's carrying Ventus's heart however; after DDD, I think they'd proly give Sora some pretty explicit warnings so that particular crap won't happen again, if not a little training if there's time. Not saying that Sora won't get more crap flung at him tho lol, if there wasn't there wouldn't be a game.