It is without a doubt Gorilla Glass. I dont see a need for screen protectors as I treat my phones very well. My OG Droid never saw a screen protector either and that screen is still minty fresh too.
Besides, most screen protectors from what I've read leave smudges all over the screen, no thanks.
If you search youtube you can find test videos for Gorilla Glass. Its very tough and scratch resistant.
Like all of Samsung's Galaxy S phones, the screen is American-made Owens-Corning Gorilla Glass (from the O-C plant in Lexington, Kentucky). What is hilarious about the glass formulation is that it's only one year younger than I am (it dates back to 1962), and thus is actually older than most G-S owners.