exa speaks the truth. Here's what I did, in a little more detail:
I have a 20" Cinema Display connected to a Quicksilver Dual 1GHz that I leave on 24/7. In Energy Saver, I set it to sleep the computer "never," but to sleep the display after 15 minutes. I then went to VersionTracker and downloaded a really simple freeware screensaver that just sets the screen to black. I configured it with a hot-spot in the lower right hand corner of the screen.
Now, I can immediately force the screen "blank" if I'm working in my home office and don't want to be distracted, and if I leave it alone for an additional 10 minutes, the back-lighting will also turn off and the monitor will happily sleep until I need to use the machine.
Voila!
-A.