For in that sleep of death what dreams may come.

[Mnemosyne]

Initial thoughts...

I'm going to have to say no on the "Mannish reincarnation" theory because Men and Elves are normally set up as foils to one another; and even if this is hobbits we're talking about, mortals are a Primary World race so I think Tolkien would be wary of including things about their afterlife that were so diametrically opposed to his Primary World faith. Elves are supposed to be bound to the world, not Men--though obviously the point about some Mannish spirits lingering weakens my argument somewhat.

I had always considered the case to be one of possession. All four hobbits in the books go through moments when they know not what they say, and then come to themselves afterwards. In the case of Sam and Frodo (in Shelob's lair) this seems to be something positive: either osanwe courtesy of Galadriel (idea stolen shamelessly from the CbC discussion) or maybe some sort of innate inner Elvishness buried deep in the subconscious suddenly manifesting itself*?

But Merry's case seems to be a whole lot closer to Pippin's experience after he looks into the Palantir: harrowing to the hobbit and creepy to the reader. In the Necromancy thread some people have briefly touched on the idea of what exactly the Barrow-wights were. Could this have been a spirit waylaid by Sauron's power on his way to Mandos and bound there?

And of course this still leaves the question: why Merry and not Pippin or Sam?



*One could, perhaps, argue Merry's dream as an inner Dunadan manifesting itself, but that makes a lot less sense because it's so darn specific; and hobbits seem to have been influenced more by the Elves directly (at least in their distant past) than by Men influenced by Elves.