A lot of people are under the impression that MiB had taken Christian's body, as he did with Locke (because both similarly landed on the island as a dead corpse) because his body was not in the coffin when Jack found it in the cave in "White Rabbit." This is not the case, if anything it's more realistic to assume that MiB has just taken Christian's form. In the FSW, we learn that Christian's body had been "lost," and therefore was never in the coffin at all. MiB or someone with similar power had just taken Christian's form, momentarily, as he did with Yemi (Eko), Dave (Hurley), Isabella (Ricardus), and the list goes on.
Just wanted to clear up whether or not Christian was actually on the island for those who really thought that MiB had taken over his corpse. More evidence pointing to MiB being Christian's appearance, and not literally in his body, is that Locke (after the Ajira flight, as MiB) was standing knee deep in the ocean. Christian was seen doing the same thing in his second appearance to Jack in the first few episodes.
This brings me to my next point: MiB hadn't found a loophole until he realized that his manipulations would never lead the LOSTies to kill themselves (because they can't, as shown with Jack and the dynamite).
In the first instance that he tried to lead a candidate to their demise, he had taken Christian's appearance, and led Jack to nearly falling off a cliff. Another example was when Dave (MiB appearing as him) was trying to convince Hurley to jump off of the cliff, because Hurley started to believe everything was in his head (this wasn't too hard for Hurley to believe in, as he had been in a mental institution before). The candidates cannot kill themselves, so this method of manipulation would never work. An example of this method working on a non-Candidate was when the MiB appeared to Shannon as Walt, and led her to being shot by Anna Lucia. This may have been a ploy to channel the anger in Sayid, for further use when MiB would have an opportunity to claim him.