Episodes 1-12 of Season 4 have further enhanced the idea that the island inhabits the dead in order to affect the living. We further have support to the notion that the islands manifestations of the dead and the actual spirits of the dead are perhaps seperate entities..each with their own agenda..or perhaps it boils down to perception and circumstance.
For exmaple, in the episode Something Nice Back Home Jack sees a very serious and haunting 'manifestation' of his dead father, Christian..he's wearing the same suit and sneakers he was in when Jack saw him on the island (White Rabbit). However, in the same epsode, Claire sees a much more jovial and 'friendly' version of Christian. He's dressed in what we might call 'other-gear' and is bouncing baby Aaron around in his arms.
Thus we have to reconcile these to disperate versions of Christian Shepherd. I think this thread is on the right track not only in terms of how the island uses the dead in order to affect the living, but also in terms of the notion that the spirits of the dead exist along side the 'manifested' version of them that the island is able to produce.
The island's version of the dead invaribly seems more serious and stern:
Whereas the possible spirits of the actual deceased, seems more true to the character before they died, nd generally more 'friendly':
Is this an example of two seperate entities battling to exert their will on the living? Or two extremes of the whole? And how/where do the whispers fit in? At this moment I would go out on a limb and say that the island is able to recreate 'versions' of the deceased via the subconscious of the living and that the dead are able to communicate with the living (via reanimation..or perhaps subconscious manifestation and via whispers.). I believe there is enough room in what we've seen for both notions to exist side by side.
As for Locke's dream of Horace (Cabin Fever) - it wasn't quite a sighting since Locke was dreaming. However, it still fits within the premise that the island can use it's cache of the dead (and their downloaded consciousness?) to direct and redirect the paths of the living. In this case, 'Horace' was used to help Locke find not 'Jacob's cabin'. I find it interesting how Horace seemed to be running on a 'loop'. Definetely an island (etc) manifestation and not the spirit of Horace..or if it was, it was a hybrid of the two entities.
Another interesting aspect regarding the usage of the deceased - Christian Shepherd acting as spokesman for Jacob. We already had a pre-curses for this when Hurley saw both Jacob and Christian in the season opener..but what does it all mean?
Personally, had Jacob not been present peeking through the window in The Beginning of the End then I would be even more suspicious about Christian and what he had done with Jacob - however, what Hurley saw (in the cabin - 4.01) could well have been a 'meeting' between the two. Perhaps Jacob summond the spirit of Christian, or perhaps he simply recreated a version of Christian because he thought his character would be the best way to approach the impending chain of events? It's difficult to tell at this stage, but it is worth noting that 'Christian' appears jovial and upbeat to everyone except his own son, Jack. Why does he haunt Jack, yet break bread with Locke? Is it partly to do with the preconception that Jack has of his father? Is the ability to 'let go' (or otherwise) partly responsible for how a person perceives the dead?