Will this question ever be answered? Who knows but in the meantime lets dig deeper into clues from previous episodes.
In "Three Minutes" Michael is being held in the Others camp by Bea Klugh, she asks him questions about Walt's childhood and about him being special. She asks "Did Walt ever appear in a place he wasn't supposed to be?"
Here's my theory, when John was in the grave and Walt appeared to tell him he had more work to do, I believe it was actually him. Part of Walt's powers include being able to appear in different places. What could this mean? Does he do it by telepathy or time travel. He did look older in the scene with Locke. Maybe this is a too far fetched idea for his powers but what do you think? Namaste.