[Boromir88]

Amras to put it in the words of Goldberry...'He is.' Which means he simply just is. As Tolkien wrote in an unpublished Letter in 1968 (and appears in Hammond and Skulls Lord of the Ring's Companion:

'I do not know his origin though I might make guesses. He is best left as he is, a mystery. There are many mysteries in any closed/organized system of history/mythology.'

Ok, so Tolkien leaves Tom B. as an enigma to us all (as I doubt he really knew 'who Tom B. was', but I highly doubt that Tom Bombadil is any character/creature/person that comes from Tolkien's Middle-earth. Because:

Tolkien creates Tom completely from a whole different story seperated from Middle-earth...not the stories he created about the History of Middle-earth. Tom B was conceived long before Tolkien got to work on LOTR or The Silmarillion. He first appears in a story about King Bonhedig, and then later on in the 1930's, Tom Bombadil is in a poem Tolkien wrote where he's traveling down a river in England.

In a Letter to Christopher Fettes in 1961:

"So Bombadil is 'fatherless' , he has no historical origin in the world desribed in The Lord of the Rings."

So even though Tom B. is in a story about Middle-earth, he is not of Middle-earth. His origin, he who is does not come from the world Tolkien created in the Lord of the Ring's. Therefor, I doubt he is a Maiar. He is a character that Tolkien wrote small pieces about in stories 'outside' the history of Middle-earth and Tolkien decided to put him into a small part in Lord of the Rings. That doesn't mean Tom B. is of Middle-earth however, that just means he was put into Middle-earth.

Again, Goldberry says it best about Tom...'He is.'