Actual metal - they're going to look just like the MacBook Pros. The white plastic is going away. I think they will be slightly thicker front-to-back (to accomodate bigger hard drives and video cards) - but without as much material around the edge of the monitor, so they will be closer to the dimensions of the screen size. 20" and 24" models only - leading to speculation that the plastic 17" iMac will evolve into the new, low cost desktop Mac for the educational market.
Hmm.