It takes a tiny amount of power to run the switches in the LCD panel. Almost all the power is used by the backlight that illuminates the whole thing. The iPad uses an LED backlight that uses less power and is brighter / pleasanter than a traditional fluorescent backlight.
I wouldn't be surprised if it turned the brightness all the way down when showing this screen, but there isn't anything else it could do to save power on the display and still show something to the user.
The explanation on the Apple website for the iPhone(/touch) says that screen brightness might drop, signal might drop etc before it goes to this screen.
Apparently Apple thought well ahead that it might be best if their device regulates its temperature rather than overheat and shut off. I'm actually exceptionally surprised by this, kudos apple.
