Pet Theory about who Tom Bombadil is

[Ibrîniðilpathânezel]

I would agree as well, and add that there is nothing to support the idea in Tolkien's writings. Gollum's "split personality" is about as close as you get, and that is nothing more than an internal struggle. In order for the Witch King to be Bombadil, he would have to somehow be able to regain visible physicality, which he lost a long time ago to the fading influence of his ring (remember what Gandalf tells Frodo about the nature of the ring of power upon mortals who possess them). If we're talking about a literal split, ala the old Star Trek good-Kirk/evil-Kirk, there is absolutely nothing in Tolkien's tales that indicate a possibility of such an occurrence. It's an interesting notion, but not one that can really work, IMHO.