I first started considering this theory when I saw the preview for next season. It says "they are not the survivors they think they are". I started wondering, what if this island IS some sort of lab experiment. BUT, it takes place sometime far into the future where we have the ability to clone people and simulate memory.

So, these people that we're watching, actually never survived the crash. What the experimenters did was they took the dead passengers' DNA and created clones of them with the same memories and placed them back on the island to see what they would do. Why? I don't know. Perhaps these people were important for some event of human history sometime in the future. Maybe it's just the first mass cloning experiment to see how accurately they are cloned.

I kinda like this theory because it explains a number of things. How Locke got his legs back (the clone never suffered the original injury leading to his paralysis). Why Jack's father was not in the coffin despite him remembering him being loaded (because Jack's father was not replicated).

I think things like the numbers that Hurley uses are crossover memories from something else - like maybe the ID number for their experiment, and so the numbers appear in everyone's memory (seat #'s, kate's bounty amount, lottery numbers, etc) in some form or another.

It also explains why someone took Claire's baby (baby mind/memory was not manipulated since it was still in Claire's womb at the time of the crash) and how Claire inexplicably escaped with the baby and shows up with just a few scratches, but with amnesia and confusion (mind/memory manipulation). Jack's father appearing could have been a flashback - an aftereffect of memory manipulation - since Kate did not see his father even though, she was in the same vicinity as Jack. The whispering heard by Sayid could either be the scientists, or it could be a hallucination (again, explainable as an after effect of mind/memory manipulation).

Some other thoughts regarding the why of this particular group of people: I'm wondering if maybe there was a crash just like this in the past, there were survivors and they did something or accomplished something that no one in the future was able to replicate or explain. So scientists in the future recreated the same conditions to see what would happen or how they did whatever they did.

Orrrr...maybe this group of people CHOSE to go on this flight with the knowledge that it would crash. Bear with me here. They ALL seem to be running from something, right? What if they decided they wanted a chance to start over, so they agreed to be killed so that they could be cloned with a chance to start over in world where everyone who knew them would think they were dead. This is a theme that constantly comes up in the show - the clean slate.