Bombadil and Istari
Kandor L.

First, in response to the earlier comment re: the Valar sending the Istari. Yes, it's true. However, each Wizard had a "patron deity" in a Valar. Gandalf was send with the Istari primarily because of the urging of Elbereth (sorry I'm rusty, don't remember High Elvish name - she's Manwe's partner) while other wizards were sent primarily because of their "sponsorship" by Aule and others. Also, yes, Bombadil could be Iluvatar (or a manifestation of him.) Here's why. Is the servant greater than the master? Gandalf could be standing next to Iluvatar and not know it, that is if Iluvatar didn't want him to know it. Eru (Iluvatar) could manifest himself in any form, anywhere in Ea without necessarily anyone (including Morgoth, Sauron, or Gandalf) knowing it. Understand? So Gandalf might not KNOW "who" Bombadil actually is. He might have a suspicion that he is a "greater power" but this doesn't prove anything specific. So sure, Bombadil could be Iluvatar (or a "piece" of him manifested in Ea) and Gandalf would still be afraid to give him the ring.

May the wind beneath your wings carry you where the sun sails and the moon walks.

By the way, this would also make feasible Gandalf's statement at the Council, "When all else had fallen, could Bombadil resist all of Sauron's power alone? I think not." (my paraphrase) This would be possible because Gandalf might not know who Bombadil REALLY is. Remember, Gandalf wandered for hundreds of years LEARNING things about Ea! He is not all-knowing, as Iluvatar IS.   ;)