Comment 4 for bug 1437180

Actually, one quick way to work around this problem would be to patch ubiquity-dm to execute this line after (or even before) launching xfdesktop:
"xfconf-query -c xfce4-desktop -p /desktop-icons/style -s 0"

http://bazaar.launchpad.net/~ubuntu-installer/ubiquity/trunk/view/head:/bin/ubiquity-dm#L530