Mine can do that but only if pressure is being applied to the 3DS while closed. Only happens if I have it in my pocket while crouching or climbing or something. Sometimes driving.
Nintendo apparently addressed this issue about a year ago though, models made after a certain date have slightly longer rubber stoppers so if you bought your 3DS a bit later you probably will never see it.
Also 3G and 4G require paying to be able to access them on the 3DS. Both Nintendo and you paying. It's not worth it
Maybe you can identify that through the model serial code or something?
I kept that little styro sheet (it was there when I bought the 3DS), and a thin wipe-cloth in between the screens, so I never had the problem. I have heard of it before I picked up the 3DS though, which is why do it.
I've heard of people doing that. It just wipes off for me so I don't worry about it. I usually carry a microfiber cloth around anyway
Is it like those DS Lites that used to have that crack? Mine's almost like this:
No Shizuka, it's just little marks on the top screen kind of like you slide a fingernail down it while wet or something. Wipes right off, doesn't do any damage to the screen.