As I've discussed previously on this blog, I have disabled and deleted Aperture previews. A drawback of this is I cannot set the screen saver to source images from Aperture directly. My workaround is a 'Screen Saver' folder where I'll export version from Aperture periodically.

Now that I've been playing with Automator more, I've streamlined the process of refreshing my 'Screen Saver' folder. A very simple service you can quickly create yourself. Launch Automator, choose Service, drag in the actions you see at right, and save. 

The primary customizations are highlighted in yellow - the folder where you'll store your photos to use in the screen saver. You may also want to choose a different export preset.

With the service installed, select your photos in Aperture and then run the service via the Aperture -> Services menu.

WARNING: The service will delete anything in the folder you've selected in Automator.

I suppose the warning is the reason I have the word "refresh" in the service title. If you are sourcing your screen saver images from an Aperture Album, then a refresh should be fine. I use a Smart Album as my screen saver source, so the album acts as an additive repository. A complete destroy and re-export does take a little longer, but I'm not refreshing my screen saver daily, so I'll take the hit.

If your selection criteria for screen saver images is more complex, not something that is captured within a Smart Album, or you're looking for an additive method at the Finder level, then this service isn't for you. You may be better served by exporting images ad-hoc from Aperture.

And just in case you don't know how to tell the screen saver to pull images from a folder, go into the System Preferences and click Desktop & Screen Saver. You can set the folder there.