It's a common desire. I need my iMac running at night so it gets backed up, but I don't want the photo show ("screensaver") running all night. On the other hand, I do like to have it run when I'm around. Problem is, nobody knew how to do this two years ago.
The best solution I found then was one of the elegant "dockables", which have been recently revived and updated (get the whole set!). The screen off dockable was quite good, but it had a few drawbacks. It couldn't be activated when the screen was locked, but when activated it left the screen unlocked. That's a problem with my kids.
Tonight, on a whim, I looked for current solution, and found How to turn off the iMac display | eHow.com.
Control-Shift-Eject. Presto, the screen is dark. It even works on a locked screen running my slideshow. (Control-Eject brings up the shutdown menu.)
WTF?! Why couldn't I turn this up two years ago? It's trivial to discover today. Was it some secret addition to an OS release? Was it always there, but only discovered a year or two ago?
The answer appears to be Yes and No. It's new to 10.5 (Leopard) and it's never been officially documented by Apple.
It doesn't appear in this 2006 article on similar shortcuts and it doesn't work on my old 10.3.9 iBook. It really only gets mentioned after Leopard is out.
On the one hand, a great feature. On the other hand, would it kill Apple to document it? It doesn't appear on Apple's recently updated screen shortcut kb article.
For me it justifies almost 10% of Leopard's purchase price. Hell of a way to run a company!
Following the trail leads to some other obscure Mac tips: