There was an article in today's Chicago Sun-Times. It talked about Louise Mingenbach who was the costume designer for Superman Returns. In the article it talks about the Superman costume and the 75 fittings it went through. It says the costume is made of an ultrasheer material that you can see through when held up to the light. She then says the material is called Milliskin ,a type of spandex blend. I did a quick search, this site had the material listed... scroll down a bit
http://www.tutu.com/ntn_fabric_stretch.html
http://www.tutu.com/ntn_fabric_stretch.html