Our trusty spudger makes quick work of the copper film found beneath the battery.

This could be a heat sink since it seems to be made of a copper alloy, possibly copper-tungsten, or copper-molybdenum. However, it is more likely that it's just an EMI shield.

Engadget remarks that the Nexus 7 doesn't get "disconcertingly hot," rather "just a little toasty".