I'm the Debian package maintainer for "floatbg" which gradually changes the desktop background colour over time, and I just hacked in Xfce support.
To avoid linking with libxfconf and having the floatbg package pull in dependencies which Gnome or plain-X users might not want installed, I'm calling out to xfconf-query (I know it's not the most efficient way...) but I wonder, is that command-line interface stable, or likely to change?
At the moment I'm using Xfce4 4.6.1 from lenny-backports.
I use the same method in scripts for random wallpapers, works fine. The API should be pretty stable as of 4.6, although some channel/properties might change from version to version. You should have included a link to your app