I've been using Xfce 4.8 since it was released. God knows what they have changed with the policy kit permissions since then, but I've never been able to get out-of-the-box standby and hibernation, or mount permissions on Debian with a non-GDM login manager. It seems like the way to fix this is simply to edit /usr/share/polkit-1/actions/org.freedesktop.udisks.policy, and /usr/share/polkit-1/actions/org.freedesktop.upower.policy.
See http://forum.xfce.org/viewtopic.php?id=6662 for more info!