There has been some debate about it, but I think that most scholars believe that it was Clement Clarke Moore who wrote it. Nancy Marshall, the author of "The Night Before Christmas, A Descriptive Bibliography," states in her book that Moore was almost definitely the author.
It is an interesting issue though and it is cool you brought it up!
I've read that Clement Clarke Moore may not have written the famous poem, but took credit for it. Some people claim that it was written by Henry Livingston - check out this site: http://www.iment.com/maida/familytree/henry/
and I've also noticed that the University of Toronto Library believes that Livingston wrote it.
In the year 2000, Don Foster, an English professor at Vassar College in Poughkeepsie, New York, used external and internal evidence to show that Clement Clarke Moore could not have been the author of this poem, but that it was probably the work of Livingston, and that Moore had written another, and almost forgotten, Christmas piece, "Old Santeclaus." Foster's analysis of this deception appears in his Author Unknown: On the Trail of Anonymous (New York: Henry Holt, 2000): 221-75. (http://rpo.library.utoronto.ca/poem/1312.html
Wow more food for thought.