we didn't actually ship gdm 3.2.1.1 in f15, so I'm assuming that's fc16 in the version string and not fc15 right?

The proper way to change the background is documented here:

http://live.gnome.org/dconf/SystemAdministrators

Basically you put a file in /etc/dconf/db/gdm.d of the form:

99-background-override

with the contents:

[org/gnome/desktop/background]
picture-uri='file:///usr/local/rupert-corp/company-wallpaper.jpeg'

then run dconf update as root.

I believe it's not instant if you use gsettings, because you would need to run gsettings where it has message bus available. The key file method above though is the DConf Approved way of making these kinds of changes though, and if you do it that way, the changes should be instant.

It's fc15, but a backported gdm in order to solve the issues in 
  
  https://bugzilla.redhat.com/show_bug.cgi?id=750827
  https://bugs.freedesktop.org/show_bug.cgi?id=42631

but you are right about versions.

The solution is almost complete, to make work, it seems like you need to have a new inode each time you want a change, so one has to delete the old file, not just overwrite it. 

Thanks!