Last time fuel got expensive Boeing looked at an aft turboprop design - 7J7. I would not be surprised if both major manufacturers waited for a Un-Ducted Fan, or Geared Fan, design to work their designs around. You can't add efficiencies, but if an airframe contributes to 15% less cost and an engine another 20 to 30% less fuel burn the combination might hit very near Boeing's stated 20 to 25% improvement.

I could not find a picture of the Boeing 7J7, but it looked very similar to the design from Long Beach.

http://www.airliners.net/open.file/0947367/L/