This is a really great tip I just found:
Access the iOS Photo Stream from the Mac OS X Finder
I recently moved my iPhoto library to an external drive because I got a brand new rMBP (yay!!) with only a 500 GB drive (I had a total of 1 TB in my old MBP). I always was using my Photo Stream for my screen saver. But if the external drive was not connected for use by iPhoto/Photo Stream, the screen saver defaulted to National Geographic pictures!
Edit: Argh! I spoke too soon. I disconnected my external drive and quit and restarted System Preferences. The screen saver preferences don't allow you to select the saved search with the Photo Stream. It's grayed out for some reason. Hmm...
Steve