The new iPod Touch is so impossibly thin that in the race to get skinny, it had to get rid of a few features: specifically, the ambient light sensor that could adjust the brightness of the screen from the brightness of the room the iPod Touch was in. That's completely gone in the new iPod Touch.
In an email from Phil Schiller, Apple's marketing honcho, he explains why the ambient light sensor was cut:
Quite simple, really. Being thin is better than being useful! [iDownloadBlog]