[Guinevere]

0riginally posted by Essex:
but isn't Frodo's journey to the West signifying his death? I'm sure someone's mentioned on this site that that's what Tolkien alluded to in one of his Letters.

Quite the contrary, Essex! (This was rather Jackson's and Boyen's idea! )

from letter 181:

The passage over Sea is not Death.
The mythology is Elf-centred. According to it there was at first an actual earthly paradise, home and realm of the Valar, as a physical part of the earth. (.....)
Gandalf was returning, his labour and errand finished, to his home, the land of the Valar.

from letter 325:

As the Ship vanished, it left the physical world. There was no return.
-The Elves who took this road and those few "mortals" who by special grace went with them, had abandoned the "History of the world" and could play no further part in it.
-The angelic immortals , the Valar or Maiar (such as Olórin = Gandalf) needed no transport unless they for a time remained incarnate, and they could, if allowed or commanded, return.
-As for Frodo or other mortals, they could only dwell in Aman for a limited time - whether brief or long. The Valar had neither the power nor the right to confer "immortality" upon them. Their sojorn was a "purgatory", but one of peace and healing and they would eventually pass away (die at their own desire and of free will) to destinations of which the Elves knew nothing.

even more detailed, concerning Frodo (and Bilbo) is letter 246:

Frodo was sent or allowed to pass over Sea to heal him - if that could be done, before he died. He would have eventually to "pass away": no mortal could, or can, abide for ever on earth, or within Time. So he went both to a purgatory and to a reward, for a while: a period of reflection and peace and a gaining of a truer understanding of his position in littleness and in greatness, spent still in Time amid the natural beauty of "Arda Unmarred", the Earth unspoiled by evil.

Bilbo went too. No doubt as a completion of the plan due to Gandalf himself. Gandalf had a very great affection for Bilbo. His companionship was really necessary for Frodo's sake - it is difficult to imagine a hobbit, even one who had been through Frodo's experiences, being really happy even in an earthly paradise, without a companion of his own kind, and Bilbo was the person that Frodo most loved.
But Bilbo also needed and deserved the favour on his own account. He bore still the mark of the Ring that needed to be finally erased: a trace of pride and personal possessivness.(...) As for reward for his part, it is difficult to feel that his lilfe would be complete without an experience of "pure Elvishness", and the opportunity of hearing the legends and histories in full, the fragments of which had so delighted him.

Too bad Tolkien didn't mention Sam in this letter... but surely he wouldn't have Sam travel to the undying lands only to see Frodo's grave! That would be too cruel!!