f you happen to own one of the new 27-inch iMacs, you may have heard that you can also use it as an external display for your other computers and devices (DVD players, for example). Although the process is as straightforward as connecting another computer to your iMac via its Mini DisplayPort port, the fact that Mac OS X keeps running in the background while it is in Target Display mode makes it just a trifle more complicated a setup than your average monitor.
Note that the instructions are for computer-to-iMac connections -- adapters and converters for other devices aren't available yet and Apple says it's leaving it up to 3rd parties to provide such adapters.)
To assist you with setting up your 27-inch iMac (owners of the 21.5-inch version need not apply) as an external display, Apple has published a support document that walks you through the simple process and helps you figure out how to operate the Mac while it is in Target Display mode. The most important tip Apple mentions is that the keyboard shortcut Command + F2 allows you to toggle Target Display mode on and off.
Your iMac needs to be on and running before you can use it as an external display. When it is in Target Display mode, the copy of Mac OS X on your iMac remains fully functional in the background. However, it does not intercept any input from the mouse and keyboard, except for the display brightness and volume controls and the media playback keys. The iSight camera and USB and FireWire ports also remain inaccessible to the external source. Check out Apple's support document for more details.