When I enter Full Screen mode, the desktop background for my second display (not the one display Aperture) changes to what I beleive is the default Lion background. Not the background I set in System Preferences.
Just guessing and trying to understand what is happening: The problem seems to be caused by two things:
An Application in Full Screen Mode will move sideways to the new Full Screen Desktop, that is a different one from all the desktops you configure in Mission Control. And this Desktop inherently has the fashionable, grey linnen backdrop, you can do nothing about it -afik-, short of patching the Appkit-Framework in the System Library:
/System/Library/Frameworks/AppKit.framework/Versions/C/Resources/NSTexturedFullS creenBackgroundColor.png
/System/Library/Frameworks/AppKit.framework/Versions/C/Resources/NSTexturedFullS creenBackgroundColor.png
But then you never would see the grey linnen again.
And - that I really consider to be a bug - an application in Full Screen Mode seems to grab all screens, not only one. So the second screen also shows the backdrop of the virtual screen.
Regards
Léonie