"I'm not a What, Ben. I'm a Who" by Ricardus Alpert

After watching "LA X", I'm convinced that during all of our speculation and discussion about The Smoke Monster- We kept asking the wrong question. It's not "What is The Smoke Monster?" It's "Who is The Smoke Monster?" And the answer to that question is Jacob's Nemesis, The Man in Black. A lot of people seem to think that MIB is another impersonation of a dead person, but I really doubt that's the case. The following exchange between Flocke and Ben convinced of it:

Ben: What are you?
Flocke: I'm not a What, Ben. I'm a Who.

I doubt that the writers would have planted that line in Flocke's mouth if it wasn't significant. Not to mention the fact that when all hell breaks loose at The Temple, Lennon tells Hurley they're taking those measures "To keep him out." "Him", not "It." There's also the fact that MIB's desire to "Go Home" seems to be so human that the character can't be just a cloud of Smoke. It's a formerly human shape-shifter, who can transform into a cloud of Smoke.