To use an external display only when watching a movie on an external TV or projector it’s nice to not show the video on your Mac’s main display since it’s distracting. Closing the lid on a MacBook Pro makes it go to sleep and the only way to prevent the MacBook from sleeping with the lid closed is through some serious kernel hacks.
Apple’s Front Row has a feature that makes the secondary screen will go blank and dark when the Front Row is started. The trick to make your main MacBook display go blank when outputting Front Row to the secondary screen is to make your secondary screen the primary display. This is done through System Preferences => Display Preferences.
When you have the DVI to Video Adapter connected to your MacBook Pro and connected to a TV or other type of external display, open up Display Preferences. Within Display Preferences, choose Arrangement. You should see two blue squares that represent each display, main and secondary. On the main display you’ll notice a bar along the top. Simply click and drag this bar from the Main Display (usually on the left and large) to the secondary display, to make it the Primary or Main display. Close Display Preferences. Launch Front Row by hitting Apple key + Esc or by hitting the Menu on your Apple Remote. Front Row will launch and turn off the MacBook main display so that you can enjoy a distraction free viewing experience for your movies and videos on an external display.