[Lord of Angmar]

I would like to propose a theory to the court of the Barrow Downs: Sauron was never physically manifest solely as a flaming eye, nor was a flaming eye ever physically manifest whose powers were derived from Sauron.

There is really not enough evidence to prove an antithesis to the above statement, as you suggested, Manwe Sulimo, with the following quote:

One moment only [Barad-dûr] stared out, but as from some great window immeasurably high there stabed northward a flame of red, the flicker of a piercing Eye; and then the shadows were furled again and the terrible vision was removed. The Eye was not turned to them: it was gazing north to where the Captains of the West stood at bay, and thither all its malice was now bent, as the Power moved to strike its lethal blow; but Frodo at that dreadful glimpse fell as one stricken mortally. His hand sought the chain around his neck.

However, let me read you another quote from Return of the King, from the "Mount Doom" chapter as well:

"I am naked in the dark, Sam, and there is no veil between me and the wheel of fire. I begin to see it even with my waking eyes, and all else fades."--Frodo

Frodo "sees," "with [his] waking eyes," a "wheel of fire," when in fact there is no reason to believe that such a wheel exists or that Frodo is actually seeing an actual physical image. This does not prove that what Frodo "saw" of the Eye in Barad-dur was only a phantom vision, but it does lend credit to the theory that what Frodo saw was a metaphorical piercing eye, not a real one. The passage, after all, only mentions that Frodo, the Ring-bearer whose burden fills his mind with images of the works of Sauron, saw the Eye, not Sam or anyone else. The former passage that Manwe provided, therefore, does not conclusively prove anything.

Tolkien uses many metaphors throughout the book, such as Gandalf's describing the reach of Sauron's arm as a figurative assessment of the Dark Lord's power, and he also describes the Witch King of Angmar as a claw on the hand of Sauron. The Eye may well be another metaphorical image, although, granted, the motif of the Eye of Sauron is more prevalent then are the aforementioned examples.

Since there are no definite facts to prove that Sauron was simply a large eye at the time of the War of the Ring, and since Tolkien himself ascribed to Sauron a physical form at that time in one of his other writings, I think it can probably be assumed that Sauron did have humanoid form. I do not wish to say that Sauron was, without a doubt, not a flaming Eye, but I personally think that the description has been taken too literally.