3.22 Through the Looking Glass - Jack (1) and Kate(2)
4.01 The Beginning of the End - Hurley(3)
4.02 Confirmed Dead - NONE (flashbacks of the 4 new members on the island)
4.03 The Economist - Sayid(4)
4.04 Eggtown - Aaron(5)
4.05 The Constant (which deals with Desmond and the scientist, so this episode doesn't have to reveal one of the 6, since is Desmond got off, he wasn't on the plane)
4.06 The Other Woman (deals with juliet, also not on the plane)
4.07 Ji Yeon (Sun/Jin, don't know about this one, I don't think they would leave without one another)
4.08 Meet Kevin Johnson (Michael(6), DING DING DING DING DING DING, here's you last #6; I don't know about Walk though, I don't think he's leaving the island)
Matter fact, I don't think Michael/Walk ever made it off. I think that they sailed about on their boat and ran into the Freighter people. Michael is Ben's spy on the boat and Locke can see vision Walt, because Walt is close to the island.
SO, every episode dealing with the losties has revealed a member of the oceanic six.
While I think that is an interesting theory DarthNeil, I don't think it's even that complicated. Dharma/Hanso/Ben/etc are somehow involved in the island. They want it to stay safe and for nobody to hear or know about it. As to why jack only mentioned 8 survivors and 6 making it off the island, he had to make some sort of deal with "Dharma/Hanso/Ben/etc" (while I don't think it's Ben, he wants to kill anybody associated with the island to keep it "safe?"). Part of this deal was to not mention anything associated with the island and to tell the outside world nothing about the real island.
Richard is one of the natives that live on the island (some people surmise that they natives are immortal. remember when ben said: "richard, you remember what birthdays are right?" Ben killed all the existing Dharma people to take lead of the natives to control the island. The natives believe, maybe stupidly, that Ben is in communication with Jacob, so the natives follow Ben. Because they follow Ben, they can leave and get back to the island whenever they want. Remember that Ben told Michael/Walt to follow a specific path off the island to get away, the same way the freighter people did to get there; so Ben knows how to get to and from the island).
As to the point where they're all dead/etc, the creators have put a stop to theories involving time travel/parallel universe/purgatory. The people are alive, in the real world, and everything has a logical and scientific explanation (at least that's what they said at SDCC). But HOW that all comes together,
if I know. My fingers hurt, I'm going to stop typing now.