Tom
The following is from a different essay than my previous link. I think that these conclusions seem make to most sense to me. They are certainly worth a read to any Tolkein scholar.
http://tolkien.slimy.com/essays/Bombadil5Theory.html#Conclusions
Conclusions
After all this discussion, the true explanation for Tom Bombadil remains in doubt. While we have narrowed the range of possibilities substantially, both the theory that he is one of the Maiar and the theory that he is a nature spirit seem quite viable. In the end, each person's decision rests largely on how willing they are to extrapolate beyond Tolkien's published words to guess at his true intent.
Identifying Bombadil and Goldberry as Maiar is a natural choice which fits all of the known texts well, and which leaves only a few mysteries about them unanswered. This is a very reasonable position to take, particularly for those who prefer not to be overly aggressive in inventing answers to the mysteries in the books.
Identifying Bombadil and Goldberry as nature spirits can provide ready explanations for most of the mysteries about them, but this is not surprising: if you make up an explanation from scratch, you can choose one that fits the facts. On the other hand, the specific nature spirit theory presented in this essay is based on a relatively simple premise; there is not much room to "fine tune" the theory to fit the texts. If previously unknown writings by Tolkien matched the "predictions" of this theory, it would be greatly strengthened. Even without such tests it remains quite appealing, at least to the more venturesome scholars of Middle-earth.
In the end, the only firm conclusion that we can reach is that Tom Bombadil remains an enigma; Tolkien seems to have succeeded after all. Even if Tom is never to be understood, I think that we have learned a bit about Middle-earth by searching for an answer, and I, at least, have enjoyed the quest.
Steuard Jensen