Interesting point. I think it would be lame if Kyuubi's power came back, would seem then that Minato's sacrifice was for nothing. I doubt it has come back, as Kyuubi is sealed in Naruto so unless he is regenerating his chakra there isn't a way for his other half of chakra to get in. If that could get in, then the half in Naruto should be able to get out (which would make the seal more of a valve than a lock).