Automount of NTFS USB partitions on Gnome fails if device name is sdb (FAT32 is OK)

Automount of NTFS USB partitions on Gnome fails if device name is sdb (FAT32 is OK)

When I plug a USB storage device (i.e. USB key) on my computer with Gnome, the filesystems available on the device are automatically mounted (in /media/<partition-label>), and a file explorer (nautilus) popup appears showing the device content.
However, this automounting does not work with NTFS filesystems.
On various forums, people suggest to set ntfs-3g SUID root :
# chmod u+s /usr/bin/ntfs-3g
This does not solve the problem, here is the popup message:
Error mounting: mount exited with exit code 1: helper failed with:
Error opening '/dev/sdb2': Permission denied
Failed to mount '/dev/sdb2': Permission denied
Please check '/dev/sdb2' and the ntfs-3g binary permissions,
and the mounting user ID. More explanation is provided at
http://ntfs-3g.org/support.html#unprivileged [=> DEAD LINK]
However, since I have all necessary rights (I'm on the "floppy" group which has read/write permission on /dev/sdb1), I can mount the filesystem myself:
$ ntfs-3g /dev/sdb1 /home/me/mymount
How can I achieve this automounting with NTFS filesystems? Avoiding, if possible, changing /etc/fstab.
I'm currently using Debian 6.0 (Squeeze).
EDIT: Also, I just noticed a strange behavior: when there is already a sdbX mounted filesystem, I plug my NTFS USB device (as sdc...), and everything works as expected, automount works. But not if the device name is sdb...