Apple published a new support document yesterday to alert users to the new Target Display Mode iMac, which allows it to be used as an external monitor for another computer. Using a cable that correctly matches the video output of this machine with that of another computer, it is possible to use it in this way and even send audio to the speakers, depending on the case.
Unfortunately, the process is not as simple as connecting one machine to another, but also not as difficult. For configuration, both the 27-inch iMac and the other computer must be turned on and out of sleep (sleep) as Mac OS X continues to run on all-in-one while the Target Display Mode is active.
After connecting one machine to another (this process requires an appropriate cable, not manufactured by Apple), the user must use the shortcut Command + F2 to enable it. The iSight camera and the rear doors are disabled, and from there the machine behaves like any other monitor, which can be controlled by the other computer to which it is connected or by a Wireless Keyboard 2009 directly connected to it.
More information can be found in the full Apple article.