This is actually an idea which a camera manufacturer could incorporate into the camera rather than as an adaptor to give full frame coverage with existing lenses. I'm surprised it hasn't appeared already. I think a camera with an APS size sensor which could take e.g. Pentax K or Nikon F lenses (manual or auto-focus) and give full frame coverage would be very popular.
Originally Posted by GarageBoy
Nothing new, check out the Nikon E3/E3S
The optics shrunk the full frame image onto a P&S size sensor