The ambient light sensor is not listed under the specs for the 5th gen iPod touch. http://www.apple.com/ipod-touch/specs.html I wonder why they removed it. It was in the 4th gen. http://www.apple.com/ipod-touch/fourth-generation-specs.html
It is a default (and useful) feature in my opinion. I always loved it because whenever I used my iPod at night, it would automatically adjust the screen brightness so that it would be appropriate for viewing...not too bright, not too dark. Same for when it was super sunny out. I'm sad if they did in fact remove it, because it will be an annoying extra couple steps if I have to go into the Settings and adjust the brightness constantly depending on the time of day.