I don't buy this completely, for a couple reasons:
1. In the Lost mobisode, "So it Begins" (which is canon, despite the fact it did not appear on the show), The Christian Apparition appears to Vincent (the dog) and tells him to go wake up his son, because he has "work to do." Now, why would Smokey feel the need to lie to a dog, with no one else watching? I suppose one could argue that Smokey has all the memories of the dead he occupies, but I don't think that would necessarily lead to Smokey referring to Jack as his son. Later, he refers to "his son" again when he bids farewell to Locke in the frozen wheel chamber: "Say hello to my son!"
2. In Season Four's "Beginning of the End", Hurley happens upon Jacob's cabin and looks inside. He sees The Christian Apparition sitting in a rocking chair, and then is startled by an eye that looms in front of him. Assuming this wasn't a hallucination, there were two separate beings in the cabin, or one being that could appear as two separate beings. We've never seen Smokey pull a trick like this before. He's either black smoke or one person, never two people.
3. In Season Four's finale, The Christian Apparition appears to Michael on the freighter. In the episode, Michael has been attempting to keep the freighter bomb from detonating by spraying the circuitry with liquid nitrogen. The Christian Apparition appears to Michael, and tells him that he "can go now," and then the bomb explodes, apparently killing Michael. If Smokey can't leave the island (per Season Six revelations), then how does he manage to appear on the Freighter?
4. In Season Five's "This Place is Death", The Christian Apparition appears to John Locke in the frozen wheel chamber (after the castaways had been time-skipping for awhile) and gives him instructions to turn the wheel and find Eloise Hawking (ex-other, Widmore's ex-paramour, Daniel Faraday's mother, Desmond's "course corrections" officer). As we find out in "LaFleur", this is some time in the past when the giant statue is still intact. Since we know now that the statue was destroyed by the Black Rock, which arrived in 1867, Locke must have turned the wheel sometime before then. Yet, The Christian Apparition refers to events that are happening in present day - How Ben turned the wheel, his past conversations with Locke in the Cabin, that Eloise is in Los Angeles. Furthermore, he knows the exact time to appear to John to give him these instructions. If Christian is Smokey, then Smokey has to be either omniscient across all time, or an avid time traveler himself. The Smokey we have gotten to know so far in Season Six does not appear to be omniscient (his group gets ambushed by Widmore's team while he is away, for one), and has not revealed knowing anything about the future to anyone. In some ways, he seems confused by some events (the Widmore ambush and kidnapping of Jin; the little blond boy with blood on his hands, AKA Lil' Jacob).
5. When Smokey (as Locke) happens upon Jin and Scary Claire, Jin calls him Locke. Scary Claire says, "that's not John, silly, that's my friend." Later, she says she's had contact with her dad and her friend. Yes, she's crazy, but she knows Smokey can appear as Locke, so one would think she'd be able to put two-and-two together and realize that Smokey could also be masquerading as her dad. Especially if they never appeared in the same place at the same time, like Michael Jackson and Diana Ross. But she doesn't, and that suggests that maybe she's seen them together.
None of these, on their own, proves that Christian is not Smokey, but when taken together, they produce at least a presumption that Christian is not Smokey (or at least that Smokey has not been every apparition of Christian).
I still think Lost is going to be explained by events orchestrated by Eloise Hawking and Christian Shepherd. They are the two that seem to know the most about what's going on.
I still think Lost is going to be explained by events orchestrated by Eloise Hawking and Christian Shepherd. They are the two that seem to know the most about what's going on.