In Response to S.H.O.'s Post
Michael B.

First of all, there are other possibilities than Bombadil being a Maia, Vala, or Iluvatar. Secondly, Bombadil was definitely none of these three. He was not a Maia because the ring had no effect on him like it did on Sauron (who was supposedly the most powerful Maia in Middle Earth) and would have on Gandalf if he had used it. He was not a Vala nor Iluvatar because his power was limited to The Old Forest and The Barrow-downs and according to Gandalf, Bombadil's realm could have been destroyed eventually if Sauron had gotten his ring back. Thus his power was more limited than the Valar and Iluvatar. The way I see it is that Bombadil is most likely one of these two:

1. (What I think is most likely) In the first book, Goldberry says to the Hobbits something like "Tom Bombadil is part of the Earth" (paraphrased - I forget the exact words). This would totally rule out all of the Ainur since they existed before Arda was created and it would certainly rule out Eru. To me, Tom Bombadil is part of the envirorment of The Old Forest and The Barrow-downs like birds, animals, trees, and rocks are in those areas. This would explain why the ring had no power over him since it wouldn't have any power over other "non-humaniod" things and beings which are part of the enviroment. I think that Tom Bombadil is something non-humanoid (and therefore not weighed down by human worries, desires, and powers) given a human-like appearance and speech. He has no fear of things that humans should be afraid of (nor does he view them as being bad - he doesn't want to destroy the old willow tree and only destroyed one barrow mound to save Frodo), he has no desire to control the world or change it for the better or worse in any way (he seems indifferent to Sauron - I doubt that the Valar and Iluvater feel that way about Sauron), and he is able to communicate perfectly with nature (better than Yavanna and the other Valar since he is part of nature and not its master).

2. He was the Witch King of Angmar. Check out the "The TRUTH about Tom Bombadil" thread in the Middle-Earth forum on this page (of The White Council) for evidence to support this. The author of it makes a very good argument although I don't support it completely.