I posted this theory on the official LOST forums a couple of days back and it received such an enthusiastic response I thought I'd re-post it here:
I was laying in bed last night, thinking about MIB and trying to puzzle things out when I was hit with a sudden revelation. It was like finally solving a riddle. The pieces came together all at once and it struck me as so GLARINGLY obvious that I just HAD to post it here.
What similarities do we see between the first plane crash and the second? There were quite a few, but the one that seems the most crucial is the fact that both planes were carrying a coffin. Notice that MIB assumed Locke's form when his body was brought onto the island in a coffin. He's parading around in his new Locke suit, manipulating people, setting things in motion for his eventual escape.
But this isn't the first time he's assumed the role of one of the dead. People have often speculated that Christian was really MIB in disguise, but if you look at the similarities between what's happening now and what happened THEN, it becomes too obvious to be a coincidence. When they crash landed the first time, certain people kept seeing Christian, who was brought back in a coffin and whose body was nowhere to be found. He seemed to have MIB's interest in mind, giving people strange advice (like telling Locke to leave the island and die) that the REAL Christian wouldn't have given. He might have been a drunk, but he wasn't a monster.
So the first time they crash, MIB assumes Christian's identity for a short period to set things in motion. He knows this form won't be good enough to do EVERYTHING he needs to do, because not everyone knew Christian and wouldn't be willing to accept him as a leader. There would be serious trust issues. He also knows (for some odd reason we still haven't been told) that he won't be able to leave the island without ALL of the candidates in tow. Since he can't put his plan into effect just yet, he manipulates things so Locke convinces the Oceanic Six to return and then gets bumped off. When the survivors return, they somehow get the strange notion that they need to bring Locke back with them, courtesy of some suspicious advice.
And that's when FLocke was born.
The only hole in this theory is Christian's missing body. I'm thinking that since MIB was using his form (like he's using Locke's now), he simply took the body off and moved it and/or buried it.
Anyway, that's my two cents. Smokey WAS Christian, that much is obvious. It makes too much sense.