Comment 16 for bug 631217

I can confirm same symptoms. they don't go away when 2nd display removed, but someone without 2 displays will need to confirm whether a 2nd display is actually needed to trigger this, or if it's just the most common reason someone would invoke gnome-display-properties. I don't want to reboot yet, but I'm aware I haven't rebooted or even re-started session since apt-get of gconf changes.

Workaround: do a SwitchUser with 2nd monitor plugged in, then switch back. I found that 2nd monitor was detected & enabled after that, so it's almost certainly an issue affecting only the session that was running when gconf-something was updated.
Perhaps a simple LockScreen will even fix it?