My intrepid readers have come together for a good explanation of the horns on Moses' face:
The first poster is right--St. Jerome's Hebrew was a little shaky & where the text says Moses' face was shining with rays of light after praying at the tent of the Lord, the Vulgate said he had horns of light. St. Jerome's translation was the one available to Renaissance artists.
There is a rather large fountain outside the Church of St. Susanna in Rome of Moses as well, and he's got horns there, too. (By the by, St. Susanna's is the American parish in Rome, administered by the Paulist Fathers. If you're in Rome and looking for tickets to the Wednesday Papal Audience, stop there. They also have Mass in English at 6:00 PM, too, if you need the break from all that Italian.)
The next trivia question:
Said statue of Moses is located in which Church and adorns whose tomb?