In the System Preferences, you can set the display to go to sleep in the Energy Saver section. However, this wasn't enough for me, so I made a small, completely black image file (1 pixel for height and width, that's all) and threw it into my iPhoto library. Then, under Desktop & Screen Saver, I set that file as my screen saver and turned on a hot corner for ease of use.
You might find that to be somewhat helpful; although, your screen will still be on, just black.
-Chasen