Well, these are the 5 I get so far.. Jack, Kate, Hurley, Sayid and BEN. We saw that Ben was off the island and looks like blackmailing Sayid to kill, he said something like "you want your friends to remain safe don't you?" Of course, Ben can't come back as Ben..how would they explain oh we found this guy on the island in the middle of nowhere? My guess is Ben takes on the identity of someone who actually was on flight 815, he has very detailed info. on everyone on the plane so it'd be easy for him to figure out who didn't have any ties in the "real world" and become that person. Since Jack lied on the stand for Kate, he said only 8 people survived the crash, etc. I believe when they actually do leave the island it is not because they want to return home but because a situation arises where they are forced to make the decision to leave, thinking they are saving everyone else on the island. So I think they are all willing to lie no matter what it takes, to keep everyone back on the island safe, so they would agree to lie about Ben's identity. My guess is when that time comes, Jack has to make a split second decision to take Aaron...maybe Claire is off somewhere at that moment and they are watching Aaron, and Jack makes the decison to take Aaron to save him, but taking him away from his mother in the process. I think this is why he can't bear to see Aaron in the flash forward, it is guilt from taking him away from his mother and now lying to everyone that he is Kate's son. I can't think of any reason why Ben would agree to leave the island, all I can come up with for them leaving is like I said, a split second decision to leave thinking they are saving everyone else on the island. Or maybe they flee..not trying to return home but get off the island, maybe to sail to the other side or something along those lines, and in that time someone turns the "protection" back on, and they can't find their way back to the island. I think Locke will remain on the island, of course, to take over Ben's "work," whatever that actually is.