I like my 3G's plastic back WAYYYY more than the stainless back iPod's have had for years. That was one thing I never liked about iPods.
As for your question, NEVER unless they somehow figure out a way to put all the antennas right behind the glass above and below the screen, but then it would probably be sending too many radio waves thru your head, so again, NEVER. If you must have a stainless back, get an iPod.