After watching 6x03, "What Kate Does," I believe that we can finally understand Christian's role in the matter... he was "claimed" by the Man in Black, just like Claire and Sayid.
Here are my thoughts:
*Christian was resurrected, like Sayid, by the Man in Black upon his arrival upon the island.
*"Claimed" Christian told Locke to move the island; "Claimed" Christian recruited his daughter to the Man in Black's side as well.
*This also explains how Christian and Flocke could both be interacting with people. Those who said that Christian was simply a manifestation of the Smoke Monster had a hard time explaining how exactly Christian was talking to Sun and Frank while Flocke was at the Ajira camp on Hydra Island.
*Therefore, although we don't understand the Man in Black's true purposes for Christian yet, I think we can safely guess that Christian has been claimed, just like Sayid and Claire. He is not just a ghost... he can grab and touch things... but he's more than just a manifestation of the Man in Black.