Apple sells roughly 10 million ipods a quarter. That's 40M in a year.
If 0.05% of them have a battery issue that will cause them to melt or burn, that is 20,000 iPods per year that will have a "thermal event" (as we say in the automotive industry). Not small potatoes, but still rare.
One in a million would still be 40 of them per year. I have owned 6 (counting shuffles and iPhones) and it hasn't happened to me yet.