Okay, some people have mentioned parts of this, but here is my smoke monster theory.
Think, most of the hallucinations we have seen on lost may be manifestations of the smoke monster. Yemi very clearly seemed to be the smoke monster in last night's episode.
The hallucinations all seem to be manifestation of guilt or other feelings that stem from the characters pasts. Jack seeing his father was a forced confrontation with his guilt over their failed relationship, the boar that chased sawyer was like his guilt for killing that man who was innocent, seeing Yemi has always seemed to be eko trying to confront his guilt that it was his fault that Yemi was killed, (and also his faith that Yemi is leading him). You might even say that Dave forced Hurley to confront the discomforts of his past, and his eating habits. Kate's horse wasn't really guilt, but it was a connection to her past that forced her to think about her guilt for killing her stepfather/father.
Eko was the only one to not feel guilt/ repentence for his past, and therefore he was killed.
Last season when Eko confronted the monster, it seemed to be scanning his past. Images from his flashbacks could be seen in the smoke. Perhaps this is one of the ways it determines what form to take.
It may also be possible that the smoke monster is the source of the whispers, and that whispers are really voices from the characters past. This is more of a stretch, but still possible certainly the whispers are also a representative of hte past, as they contain lines from the losties flashbacks. In outlaws, a whispered "It'll come back around" was in his flashbacks, and before Shannon dies a whispered "Dying sucks" was said by boone in her flashback. The whispers are simply the voices from the character's pasts.
Theory by losty09
