If you look at the back of the iPod touch, there is a black plastic section. That is where the wifi chip and bluetooth chip are. The first gen iPhone did that as well, the entire bottom quarter of the back was black plastic, but I guess Apple still felt the aluminum was messing with the cell signals, hence the reason they changed it to all plastic only for the iPhone, not the iPod touch.
Thanks for the detailed explanation. I was replying to someone who seemed to believe Apple had switched from the aluminum back to the plastic one because they thought the aluminum was a bad design. My point was that there was more to it than just aesthetic differences.