As pointed out in our iFans discussion forums, it appears that Apple has removed the ambient light sensor from the fifth-generation iPod touch. On the technical specifications page for the new iPod touch, the ambient light sensor is not listed, leaving the three-axis gyro and accelerometer as the only sensors in the device. Since we do not have a fifth-generation iPod touch on hand to confirm this discovery, there is still a possibility that Apple may have merely forgot to list this specification on the product page.
The fourth-generation iPod touch and previous models have always been equipped with an ambient light sensor, which is used to determine the natural lighting level and automatically adjust the brightness of the display if necessary. Typically, this sensor has been placed on the front bezel of the iPod touch above the display, although the front-facing camera currently occupies this space on the new iPod touch. We have reached out to Apple for comment and will update this post if we hear anything back.