Actually I was in a apple store a few months ago and someone asked a genius why they were getting rid of the metal back. He said that the metal blocks signal and makes downloading slower so I think its safe to say that apple will not be putting the metal backing on a phone anytime soon.
Yep, thats true. Thats why the metal-backed iPod touch has the back thing in the top left, so it can get a (crappy) WiFi signal.