It's possible that Bombadil is not affected by the Ring because he doesn't want anything. Gandalf wants Sauron gone, and ME free; Sauron and Saruman want power; others simply want the Ring itself. Bombadil wants nothing that he does not have, that's pretty clear. He's happy where he is.
But I admit, the idea that Bombadil is a Maia is almost as weak as some of the crazier ideas about Bombadil. (Bombadil is Iluvatar, Bombadil is Tolkien himself, Bombadil is a hallucination, Bombadil is the Witch-King of Angmar... every one of these theories has its proponents, you know.)
Not the Witch-King of Angmar! Bad guys don't help out good guys or good hobbits.
It's possible that Bombadil is not affected by the Ring because he doesn't want anythingYou are both straying too far, but then I brought up Ents, so we are all in tooks corner today...
But as long as you bring it up, I like this interpretation.