Mine got really badly scratched and I'd only had it for 2 weeks - I can't work out when or how because I thought I was being really careful with it - always keeping it in the sock etc.
I called to complain and ask for advice and stressed that it was my first iPod and I was always a loyal Sony lover until recently. They told me to send it in to them, they would take a look, and if they thought it was bad they would replace it, and if it wasn't, they would send it back to me.
I finally managed to get into the apple site and it tells me that a replacement is being ordered and an 'order has been created' so think I'm getting a new one.
I ordered a cheap plastic skin from decalgirl, and although its just two little clear stickers, and leaves the nano looking not as pretty and sleek as it should be (it doesn't adhere perfectly), but it has been doing a great job of keeping scratches far far away!