Spider web scratch. It's very faint but now that I've seen it, it's extremely noticeable in the light.
I had my iPhone 3G for 2 years with no case other than a leather belt holster and the screen has not one scratch on it.
There have been the same stories back and forth since the first Iphone came out. Someone hit their screen with a hammer and not a scratch. Someone else blew their screen a kiss and it shattered into a million pieces. It's unexplainable.