I'm still covered by my warranty for a few more months, but I'm lucky I have my serial number here. The little plastic sheet fell off, and a few days later I realized the serial number was fading away quickly. Using some bright light and the info on the Nintendo website regarding the prefix letters, I was able to salvage it a few days before it faded entirely. Then again, since it's not visible, they might require a receipt, which I sent off to get $16 off a miniSD card that I bought at the same time, so I could just be equally screwed if something goes wrong.