Here's something you don't see every day: a MacBook Air with a Retina display sitting on top where you'd normally see the Apple logo. The geniuses at Adafruit hacked together a Display port to Retina display adapter, using it to drive the same display that's used in the third-generation iPad. In the video below at about 49m20s, you'll see Adafruit's Limor "Lady Ada" Fried and Phillip Torrone talking about the display adapter as part of last Saturday's "Ask an Engineer" video podcast.
The adapter and display will both be available for sale from Adafruit in the near future for your hardware hacking fun. Until then, you can dream about how you'll use that "display on the back of your display" on your MacBook.