I just learned how to subvert the giant 27-inch iMac in our reading room and turn it into an external display for my MacBook Air. This is documented in the support area of the Apple site:
- Make sure that both computers are turned on and awake.
- Connect a male-to-male Thunderbolt cable to the Mini DisplayPort on each computer.
- Press Command + F2 on the 27-inch iMac keyboard.
- To leave Target Display mode, press Command + F2 on the keyboard of the 27-inch iMac that is in Target Display mode. To return to Target Display mode, press Command + F2 again.