This problem recently appeared on my computer, and I'm pretty sure it was upgrading xfce4-session from 4.4.3-0ubuntu1~intrepid1 to 4.6.0-1ubuntu2 that did it. This also swapped xfce4-mcs-plugins out for xfce4-settings. xfdesktop4 wasn't touched.
This problem recently appeared on my computer, and I'm pretty sure it was upgrading xfce4-session from 4.4.3-0ubuntu1~ intrepid1 to 4.6.0-1ubuntu2 that did it.
This also swapped xfce4-mcs-plugins out for xfce4-settings. xfdesktop4 wasn't touched.