[cutterjohn42]

-rws--s--x 1 root root 12158 2009-08-10 21:22 /opt/calibre/calibre-mount-helper

Hmm looks OK(I think). Let me try mounting the the PRS505(what I have) again and I'll update this post.

This time:
Dialog

Cannot unmount the volume 'Sony Reader Storage Card'.

details

umount: /dev/sdd1: not mounted

(First time complained about mtab entry)

a second dialog pops up:

Unable to unmount Sony Reader Storage Card

DBus error org.freedesktop.DBus.Error.NoReply: Did not receive reply.
Possible causes include: the remote application did not send a reply,
the message bus security policy blocked the reply, the reply
timeout expired, or the network connection was broken.

mtab

/dev/sda7 / ext3 rw,relatime,errors=remount-ro,commit=600 0 0
tmpfs /lib/init/rw tmpfs rw,nosuid,mode=0755 0 0
proc /proc proc rw,noexec,nosuid,nodev 0 0
sysfs /sys sysfs rw,noexec,nosuid,nodev 0 0
varrun /var/run tmpfs rw,nosuid,mode=0755 0 0
varlock /var/lock tmpfs rw,noexec,nosuid,nodev,mode=1777 0 0
udev /dev tmpfs rw,mode=0755 0 0
tmpfs /dev/shm tmpfs rw,nosuid,nodev 0 0
devpts /dev/pts devpts rw,noexec,nosuid,gid=5,mode=620 0 0
fusectl /sys/fs/fuse/connections fusectl rw 0 0

mount

/dev/sda7 on / type ext3 (rw,relatime,errors=remount-ro,commit=600)
tmpfs on /lib/init/rw type tmpfs (rw,nosuid,mode=0755)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
varrun on /var/run type tmpfs (rw,nosuid,mode=0755)
varlock on /var/lock type tmpfs (rw,noexec,nosuid,nodev,mode=1777)
udev on /dev type tmpfs (rw,mode=0755)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=620)
fusectl on /sys/fs/fuse/connections type fusectl (rw)

I've got an old Sony Main memory directory in media which wasn't removed before(have to clean that up), and a new one with an appended <space> and (1)

Trying to un mount main memory dialog:

Unable to unmount Sony Reader Main Memory (1)
umount: /media/Sony Reader Main Memory (1) is not mounted (according to mtab)

ls from CLI shows contents of both, so they're mounted.

Interesting:
/proc/mounts

rootfs / rootfs rw 0 0
none /sys sysfs rw,nosuid,nodev,noexec 0 0
none /proc proc rw,nosuid,nodev,noexec 0 0
udev /dev tmpfs rw,mode=755 0 0
/dev/disk/by-uuid/d08275b0-f5c1-4e7f-b128-caa93354ef3f / ext3 rw,relatime,errors=remount-ro,commit=600,data=ordered 0 0
tmpfs /lib/init/rw tmpfs rw,nosuid,mode=755 0 0
fusectl /sys/fs/fuse/connections fusectl rw 0 0
varrun /var/run tmpfs rw,nosuid,mode=755 0 0
varlock /var/lock tmpfs rw,nosuid,nodev,noexec 0 0
tmpfs /lib/modules/2.6.28-14-generic/volatile tmpfs rw,mode=755 0 0
tmpfs /dev/shm tmpfs rw,nosuid,nodev 0 0
devpts /dev/pts devpts rw,nosuid,noexec,gid=5,mode=620 0 0
/dev/sda4 /media/New_Volume fuseblk rw,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
/dev/sda2 /media/windows fuseblk rw,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
/dev/sda5 /media/misc2 fuseblk rw,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
securityfs /sys/kernel/security securityfs rw 0 0
binfmt_misc /proc/sys/fs/binfmt_misc binfmt_misc rw,nosuid,nodev,noexec 0 0
gvfs-fuse-daemon /home/moylan/.gvfs fuse.gvfs-fuse-daemon rw,nosuid,nodev,user_id=1000,group_id=1000 0 0
/dev/sdd1 /media/Sony\040Reader\040Storage\040Card vfat rw,nosuid,nodev,uid=1000,fmask=0077,dmask=0077,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,flush 0 0
/dev/sdb /media/Sony\040Reader\040Main\040Memory\040(1) vfat rw,sync,nosuid,nodev,noexec,uid=1000,gid=1000,fmask=0177,dmask=0077,codepage=cp437,iocharset=iso8859-1,shortname=mixed,quiet,utf8 0 0

So it looks like /etc/mtab is NOT being properly updated now, typo? Or actually it looks like fuse doesn't update mtab either, so maybe you're relying on mtab when it appears that /proc/mount should be the source?