THE RING LADY SAYS, "if you don't do those things Demond David Hume, every single one of US is dead." she emphazises US which included desmond, so i do not think desmond died. i believe he has those side effects, like seeing the future which might end up saving the losties. everything fits a purpose. death on the island, is just that. i don't believe you can die then walk the earth. the only way desmond could be dead is if the black smoke is projecting desmond to everyone (but the black smoke usually does that to one person's thoughts, not the full camp at once) so i don't think at all that's what's going on.
if he were dead, he would have stayed with penny and not follow the same steps he took..now he would have died had he not turned the failsafe key or done something different, but he stayed along his path...
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