i'll keep this brief, cos its been done to death
-christian was in the cabin which we know was not occupied by jacob, and the ring of ash strongly suggests MIB was in there -christian was absolutely pivotal to the MIB's loophole -christian is dead, the smoke monster appears as dead people- we havent seen anyone or anything else do that -when sun and frank arrive on the outrigger from the hydra island after 316 crashes, they see trees move and we clearly hear the ticka-ticka smoke sound, then they proceed to the barracks where we see... Christian, why would they show the smoke instantly before christian appears. as if that wasnt enough, Christian then tells them to wait for John Locke, who we KNOW is the smoke monster -christian appeared off island after jack fixed the SMOKE alarm (slightly tenuous) -introducing a new 3rd entity now would be "deus ex machina" (ie a lame afterthought)
now then- there are 6 pretty decent reasons to suggest that christian is an apparition of the smoke. if anyone can come up with 6 reasons why he isnt, or even just 1 GOOD reason, then i'd love to hear it