[Darv]

I'm ressurecting an ancient thread because re-reading the Silmarillion kicked off a train of thought.
Bombadil is a lesser Valar, i.e. he is an Ainur confined to the circle of the world.
I take my arguement for this from "Among them Nine were of chief power and reverence;"(p32) implying that there were others of lesser power and reverence. Certainly the sudden appearance of Tulkas to oppose Melkor argues that other Ainur would incarnate at need.
He is still working(p19) on the "minute precision"(p19) part of the shaping; perhaps looking for more things unknown to the Ainur?
This is based on the most slender of evidence but it would explain his immunity to the ring; he is from outside ME and so not subject to its laws.

I'm using the 1979 Allen and Unwin paperback Silmarillion.