cycas:

Ooh!  what an interesting question.  Sadly, I don’t have a super-interesting answer for it though. 

I think Tom Bombadil is Himself.  I don’t think he’s someone who is mentioned in the Silmarillion or HoME under another name.   I don’t think he’s Eru.  I don’t think he’s a Maiar or a Valar or anything described and categorised elsewhere.   There’s lots of room in Middle-earth for things that are only mentioned in one place in passing: think of Beorn the skin-changer and the giants of the mountains in the Hobbit!  Not to mention the were-worms…

I’m pretty sure Tom isn’t Maglor, as in a theory I’ve seen a couple of times: I think Elrond’s reaction to the mention of him at the Council proves that: Elrond knows Maglor, of course, and he also knows Tom Bombadil as Iarwain Ben-adar, oldest and fatherless.  It would be odd for Maglor to be described as ‘fatherless’ since his entire fate was bound up with his father’s. 

Probably the most precise concept I have of Tom is as a spirit of the land: something that is a self-aware part of Arda itself.  (I tend to think the Rivers have similarly self-aware aspects, or at least the River Sirion did, and the Withywindle  (though possibly Goldberry is the Withywindle, and she’s the daughter of the Baranduin).   Old Man Willow might be something similar, if he’s not a Huorn or a darkened elf-spirit infesting a tree. 

Tom Bombadil’s self-imposed boundaries that he won’t pass are rather fascinating, aren’t they?  I’d love to know exactly where they lie and why he won’t cross them! And also his friendship with the Maggot family, who of all the hobbits we meet, seem to be the ones most entrenched in one place.