I recently found, that the automount feature of my recent Debian Sid / GNOME 3 system didn't work anymore. For example: I plugged in an USB stick. Nothing happened, although
dmesg clearly showed, that the system recognized it. After manually mounting and unmounting the device, nautilus automatically offered to automount it. But I always had to manually mount and unmount it first. Seems the culprit was a pmount installation. JFTR: In some cases also usbmount might cause trouble.