[gentoo-amd64] Unable to mount USB and CD on Gnome

[Clemente Aguiar <ca-lists@madeiratecnopolo.pt>]

For a while now when I insert a CD / DVD or a USB pen drive on my PC, it
will not mount, and GNOME gives an error.

If it is the DVD the error is as follows:
Unable to mount "DVDLABEL"
Not Authorized

If it is the USB pen drive the error is similar:
Unable to mount File System 1,0GB
Not Authorized

If I login as root then there is no problem, the disks will mount (and
automount correctly), but if I login as a "normal" user it will not
automount, and even if I go through the menu and select the drive it
will not mount giving exactly the same error.

One interesting note is that I can mount and unmount using gnome-mount
and gnome-unmount utilities, for example:

$ gnome-mount -d /dev/sdb1
$ gnome-umount -d /dev/sdb1

Can anybody help me with this problem?