Now that the R800 mystery was solved, I carried on my merry way customizing the system to my tastes. One of my idiosyncrasies is to place all of my personal background images in their own folder in the Pictures folder. That way I can easily use a special Export Preset in Lightroom to easily create background (wallpaper in Windows terms) images and not have to fiddle with System folders.
In the System Preferences there is a control called Desktop & Screen saver whose job is to control desktop background images and screen savers (doh!). However, my setup was listing Aperture 2 folders twice and any attempt to add my Backgrounds folder resulted in the system informing me that it was already there.
Relying upon my mad Google-Fu skills, I uncovered that this problem can be solved by opening iPhoto once. This recreates the list adding the iPhoto Library to it, removing the duplicate entry for Aperture 2 and exposing the folders already added to the list.
Simple solutions (however odd they may seem) are always the best. I am also including a screengrab of the way the control should look after you run iPhoto for the first time.