At first I believed Christian Sheppard was resurected due to the fact that Locke had to play the role of Christian on 316 and Locke was resurected. Christian also wasn't in his coffin when Jack found him, so there is another hint there. It just doesn't make sense to me though that Christian can be at several places that wouldn't seem reasonable (like the freighter?). I know have a feeling he may be the monster just taking the form of Christian as well. I don't believe he could be Jacob because it just doesn't really make sense, but this is Lost were talking about! So I either think he may be resurected or he is the monster, not sure though.