Walt is "special," it's probably the reason the Others took him. We already know that psychic phenomena happen on the island from seeing Jack's dad, and Boone's vision of Shannon's first death. (Death sucks) We know from the Dharma Initiative orientation film that the Island is a large scale communal research compound (gone bad) consisting of 6 stations studying Meteorology, Psychology, Parapsychology, Zoology, Electromagnetism and Utopian Social (unintellegible.)
Get your Orient on.
Anyhoo, spooky things happen on the island; wet, creepy Walt is not Walt literally walking around the island talking backwards, but it is Walt, wherever he is, communicating with the Losties.