Cannot suspend / hibernate within xfce

[mittfh]

So within the xfce desktop, if I click on the Action button and choose Suspend, Hibernate or Shut down from the menu, after clicking the confirmation dialog, I'm then presented with another one:

Shutdown failed
Failed to suspend session
not authorized

It appears to be a desktop-specific permission, as if I login as me to one of the ttys and type "pm-suspend" or "pm-hibernate" it's perfectly happy to do so.
I'm guessing enabling these actions may be as simple as editing a configuration file... somewhere. But if so, which one?