The SGP screen is indeed glass...Gorilla Glass...made by Corning, I believe. It is harder and smoother than regular glass, so it resists scratching. It CAN be scratched, but not as easily as standard glass. It also resists cracking. It CAN be cracked, but not as easily as standard glass.
If you're one of those people that is careful with your devices, you will likely not have any damage issues. If not, you might want to get a screen protector to help prevent scratching. Personally, I don't use one.