In a deleted scene for season Four episode Nine The Shape of Things To Come, Claire and Hurley are in the Dharma house bed room after Claire's house is blown after the merceneries attack. Claire then asks Hurley what her father is doing in the room. Theres no there. This was before Hurley could see dead people. And everytime MiB has taken the form of the dead he was never only visible to certain people, everyone can see him. I know its a delted scene but it could be proof Christian is a ghost on the island rather then just MiB.