I was thinking about this after my brain reassembled itself after last night's episode. Hurley has the same ability to talk to the dead as Miles does. This is based off of the fact that Locke was SO surprised that Hurley could see the cabin. And with Miles they've already shown that its a possibility.

Think about this... Hurley was in a mental hospital twice, and both times talked to people who 'weren't there'. One of them was Charlie, who we KNOW is dead at that point. The other was Dave, who only appeared to him when Libby was around him (both in the psych ward and on the island). Libby's husband's name was Dave, who died (which is probably why she was in the mental hospital anyway).

I think that we're made to think that Hurley was crazy, but I think we all agree that's not true. So then the other explanation is that he IS seeing these people, because he has the ability to see these beings. I realize this is pretty thin and is all speculation, but what the hell.