[Arien]

The ring doesn't affect him because he's the author. In Middle Earth, Tom Bombadil is Tolkien. The entire sequence is written, to me at least, in a way that seems to take the characters out of the story for a moment to another place. I think the cabin and Tom Bombadil is where Tolkien retreats to in his mind to create worlds such as Middle Earth. I always felt when reading FOTR that Tom Bombadil was almost a small joke by Tolkien. A "Hey, I put myself in the story" sort of thing.

That´s interessant ^^. I don´t have known this, but it sounds plausible ^^
mhh... I think that´s the best theorie of "who or what" Tom Bombadil are...
It´s improbably, that Tom is Eru Ilúvatar or one of the Valar. (<-- this possibility is impossible ^^ because there are only 14 without Melkor. ^^)
He isn´t one of the Maiar, because he doesn´t show any reaction, when he put the ring on. (Unlike Gandalf, who wouldn´t put the ring on because he was afraid of beeing hung up on the ring.)

(sry my english is so bad ^^)