Ahh LOST, we can guess and guess and guess, and we'll still be wrong.
I think that in order to be able and/or allowed to leave the Island, the Oceanic 6 agree to something with someone. We don't yet know what they've agreed to or with whom they've agreed to it. Nor do we know the ramifications of what would happen if they break their agreement.
I think that the 6 agree with someone, most likely Ben or Widmore but probably Ben and/or Richard Alpert, that in order for at least 6 of them to leave the Island they have to lie like rugs....for the rest of their lives.
I would bet that somewhere in this laying down of the lies, there is an explicit or implicit threat to those left behind that if someone lets loose with the truth, something awful will happen to those left behind.
I think those left behind on the Island are there under some sort of duress or in some sort of discomfort or disability. Perhaps that is why Charlie keep visiting Hurley telling Hurley that "they need your help."
How the 6 get together to get off the Island is open to endless conjecture. As Jim pointed out, everyone is separated from one another at this juncture. We have no way of knowing how they will come together.
I no longer think that Kate is up to something evil with Aaron, though I was majorily creeped out when I first saw them together so many episodes ago. I think that Kate winding up with Aaron was probably the most humane thing to do because the survivors probably agreed that it's best to get the baby off the Island, and, apparently, Claire is still absent when the time to leave the Island arrives.
Someone mentioned they thought that Claire was told she was not to raise the baby. No, quite the opposite. Over and over, the psychic told her that she MUST raise the baby, that the baby would be in danger if he were raised by another. At the time, it looked like the psychic was referring to The Others (Ben, Ethan et al) from Season 2. Now it could mean something else. But it seems The Island has had a hand in finally separating Claire from her baby. Ben and Ethan and all the Others couldn't do it. It took an almost direct intervention by Jacob in the form of Claire's and Jack's dad to find a way to make her leave her baby, and we don't yet know how that happened.
Oooh Oooh Oooh! I just had a brainstorm!
The method by which Locke will move The Island (most likely move it in time) will involve the not-quite-dormant-volcano on the Island, the volcano that was referenced by Horace's schoolteacher wife in the flashback with young Ben so long ago. If the volcano is involved, and I bet my paycheck that it is, then it's not a far jump to see how a cyclone could be generated by whatever force is invoked by Locke. Wasn't a cyclone (a hurricane in the southern hemisphere) responsible for supposedly washing up a boat loaded with supplies that helped the Oceanic 6 survive on their fictional desert island? Such a meteorological event is something that would actually have had to happen in the real world if they reference it in their "big lies story."
OK, let's see how well I do with THAT theory.