The following hint is useful to me because I use my iMac in my bedroom, and I don't shut it down or put it to sleep. I prefer to sleep in the dark. Additionally, I don't want people hopping on my computer and changing my stuff - so I need the password protection a screen saver provides.
Since 10.5 was released, my workaround has been to set Energy Saver to sleep the display after a certain amount of minutes, and set a screen saver to start one minute after the display sleeps. It didn't always work, unfortunately.
Here's my new solution. For this to work properly, you're going to need to have either a hot corner set up to activate the screen saver, or have the screen saver set to a timer. It's a good idea to disable Energy Saver's display sleep. Then, download and install Sleep Display and ScriptSaver.
First, write yourself an AppleScript that contains this one line:
do shell script "open -a SleepDisplay"
Save it somewhere safe. I made a folder called ScriptSaver Scripts in ~/Library/Application Support/.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090726183804718