In an interview, Robert Plant stated plainly that "Stairway" had ABSOLUTELY NOTHING to do with The Lord of the Rings. However, I am convinced that he is not being completely honest:

There's a feeling I get when I look to the West

And my spirit is crying for leaving.

In my thoughts I have seen rings of smoke through the trees

And the voices of those who stand looking...

As previously mentioned, Gandalf and the other wizards arrived in Middle Earth . from Valinor (the sacred lands of the Valar, banned to mortals) and like the Elves, they would return when their work on Middle Earth was complete. However, the beauty of the land captured the hearts of all who set eyes on it, forever leaving them yearning to return. Gandalf's hardest tasks befall him during the War of the Ring; even Frodo notices that he looks older, more worried, and even more careworn than ever. I believe this is what Plant is referring to in this verse. Also, Gandalf is famous for his colored smoke rings, and "those who stand looking" are more than likely the Dunadan (Rangers) who patrol the land in disguise, protecting it from evil. (The Simarillion p. 299-300; 102; TheFellowship of the Ring p. 54; 55)

I wrote the above about two years ago and after reading  the trilogy again, I have found even more evidence for Tolkien in "Stairway." The above still holds true, but in a more specific context, the lines above are from The Hobbit where Gandalf, the thirteen dwarfs and Bilbo are coming down from the Misty Mountains after murdering the Great Goblin. The party makes their way to the home of Beorn, the Skin Changer on the edge of Mirkwood. Bilbo more than once wishes he was back home (the West) over the Misty Mountains. Which corresponds to the lines above. Leaving Beorn's house, they are given ponies and a horse, for Gandalf to ride, to the gates of Mirkwood. Beorn loves his animals and indeed, he would let no harm come to them. He and his creatures are seen by Bilbo off to the side of the trail, watching them at night in secret. Upon reaching the gate, Gandalf confirms Bilbo's suspicions. These are "Those who stand looking."

In my thoughts I have seen rings of smoke through the trees

And the voices of those who stand looking...

A new day will dawn

For those who stand long

And the forests will echo with laughter

 
Plant could also be referring to the Woodland Elves who make their home on the skirts of Mirkwood, opposite that of Beorn. While in Mirkwood, day was as night with no light whatsoever. Bilbo could not even see his hand before his face, but he could see eyes. Big eyes, little eyes, and eyes of all colors. Toward the end of the path through the forest, they came upon the elves hunting for deer and rabbits, with big bonfires three times. They could hear their laughter and see them glittering in jewels and hear their songs, but each time the company approached, they would flee.

Finally, there is a passage in The Hobbit (A Warm Welcome) where Tolkien writes of the men of Dale,

Some sang too that Thror and Thrain would come back one day

and gold would flow in rivers, through the mountain gates, and all that

land would be filled with new song and laughter.

This little passage may not be related to Led Zeppelin, to say it does would be a stretch. However, I think the lyrics to "Stairway to Heaven" bear the same message of hope as the passage above. After all, Plant says himself "This is a song of hope."