Seems I heard somewhere that it's a gene in your body that allows cancer to happen. Some people have the gene and some don't.
Anyway, I would say that the 3DS getting slightly warm is normal, but if it gets extremely hot, as someone said theirs does when playing and charging, then I would kinda worry about that. Especially if it gets warm to the point of being uncomfortable to the touch. I highly doubt that is something Ninty ever intended to happen.