Here is my thing...
[pointless theory]
With all the mystery surrounding "whatever the hell that thing is on the island", do you think that those pirates that took Walt are actually the "Others" that the French woman was talking about? Were they "sent" by the island? When the Siaed(sp) and Charlie confronted the french woman on the beach, didn't she say something about "all they wanted was the boy?" Well, maybe they came back for another 'boy' , and Walt was the only one. The island had to go get the only boy near it.
[/pointless theory]
And now I'm done.....