Old Tom bombadil is a...?!?!

[the guy who be short]

I return to you with a new theory.

True, Bombadillo could be any of the mentioned above (well, actually, no, I'm about to disprove you all... here it comes!) but then, why does the Ring not affect him?

Merry would surely be affected by a ring, as would the Witchking or Fredegar.

No, you see, Tom Bombadil is... Eru Iluvatar. Yes! It makes complete sense.

Observe: The Ring was forged by Sauron, one of the greatest of the Maiar. It affects him. Therefore, it must affect all other Maiar as well as lesser beings. This means that no creature save one of the most hallowed could be unaffected! But the Valar were all in Valinor, meaning that Tom must be a physical manifestation of Eru himself.

Eru came to Arda to spread his Truth - that is, to preach the advantages of Nudity to all that passed by. Unfortunately, due to the hobbits' later adventures, they completely forgot to introduce nudity to the Shire, and as a race they were forever condemned to the Void after Death...