Phil Schiller, in a purported email:
The fifth generation iPod touch does not have a built-in automatic light sensor (it’s just too thin!)
This is a usability fail for Apple. The ambient sensor, especially coupled with iOS 6’s greatly improved auto-brightness feature, is a detail that makes an iOS device nice. Not necessary, but nice. You’d be smart to note that the camera stick out the back of the fifth-generation iPod touch, so perhaps Schiller’s explanation holds water. Then again, maybe the new touch really is too thin.