iPhone 4 glass (front and back) scratches very easily. If it is so-called "scratch resistant" it is only a bit more so than 3G or 3GS.
I was extremely careful with my iPhone 4 for the first 24 hours. Only went in an empty pocket on the way to work, then sat in a cradle at work the rest of time. At home it was mostly in my hand but when it wasn't it was lying on its back. I was almost obsessive about not sliding it on any surface and only wiping with a soft lense cleaning cloth. By the end of the second day, it had a nice scratch right across the edge of the screen. Not noticable when the phone's screen was active and viewing from straight on but very noticable when it was off or light was reflecting at any angle. I don't know exactly what caused the scratch but given the fact that I was so careful, I would have to say that the scratch resistant claim is mostly BS. This glass WILL scratch and it doesn't take much. Go skinless/caseless at your own risk.
There is a good ending to this story. I went to Best Buy immediately after seeing the scratch and bought an Invisible Shield for front and back. Once applied the scratch can not be seen at all in any angle of light. No big deal now. You barely notice the skin. I had planned on getting one anyway as all portable devices will scratch eventually due to the way they are used. I just wish I would have done it immediately, but since I can't see the scratch at all now, I am happy.