Neither Friend Nor Family

Just watched the "annotated" season finale of Lost from last time around, and I'm going to go on the record guessing that the body in the coffin is Juliet. Or Ben, but probably Juliet.

Why? It couldn't be one of the survivors. Not even Locke. They're still friends, despite everything that's come between them. That leaves someone from the island. Rousseau telegraphed the "I'm not going back, because I have nothing there to go back to" in the episode, but that was before she met Alex. Another red herring. And while Kate certainly hates Ben, the visceral reaction she had when Jack said he thought she might go to the funeral had a jealous overtone to it.

That leaves Juliet. Jack and Juliet don't have the shared experience that Jack and Locke have, so a betrayal would expose any hint of friendship as a lie. What's most likely -- and the "notes" on the screen may have tipped this off -- is that Juliet used Jack to get off the island, knowing full well that the survivors were meant to stay, ripping holes in the space/time continuum and all that good stuff in the process.