The pre-Retina unibody MBPs were made out of CNC machined billet aluminum, but what about the Retina series? Have they gone back to plastic for the housing? I can't find any information on the Retina MBP construction; just the usual technical specs.
Yes. They have not majorly changed the materials or manufacture of the MacBook Pro chassis since the unibody redesign (except for anodizing the new Space Gray models).
Pretty sure that Apple still refers to the chassis as a unibody design.
I imagine that it would be challenging to have one that thin, and not have a machined case like that.