I can confirm this bug as well.  I have tried a USB-IDE enclosure, a thumb drive, two different cameras in UMS mode, and an MP3 player in UMS mode.  They mostly have vfat filesystems, though I have tried ext3 as well.  *ALL* of them automount just fine under Edgy.

If I run "udevmonitor -e" while inserting my Kingston DataTraveler 1gb flash drive, with one ext3 partition and one vfat partition, it reports that correctly:

UDEV  [1191447365.558892] add      /block/sdb/sdb2 (block)
UDEV_LOG=3
ACTION=add
DEVPATH=/block/sdb/sdb2
SUBSYSTEM=block
SEQNUM=2829
MINOR=18
MAJOR=8
PHYSDEVPATH=/devices/pci0000:00/0000:00:13.2/usb1/1-6/1-6:1.0/host6/target6:0:0/6:0:0:0
PHYSDEVBUS=scsi
PHYSDEVDRIVER=sd
UDEVD_EVENT=1
DEVTYPE=partition
ID_VENDOR=Kingston
ID_MODEL=DataTraveler_2.0
ID_REVISION=1.00
ID_SERIAL=Kingston_DataTraveler_2.0_0000015955-0:0
ID_SERIAL_SHORT=0000015955
ID_TYPE=disk
ID_INSTANCE=0:0
ID_BUS=usb
ID_PATH=pci-0000:00:13.2-usb-0:6:1.0-scsi-0:0:0:0
ID_FS_USAGE=filesystem
ID_FS_TYPE=vfat
ID_FS_VERSION=FAT32
ID_FS_UUID=46E1-B3BA
ID_FS_UUID_ENC=46E1-B3BA
ID_FS_LABEL=
ID_FS_LABEL_ENC=
ID_FS_LABEL_SAFE=
DEVNAME=/dev/sdb2
DEVLINKS=/dev/disk/by-id/usb-Kingston_DataTraveler_2.0_0000015955-0:0-part2 /dev/disk/by-path/pci-0000:00:13.2-usb-0:6:1.0-scsi-0:0:0:0-part2 /dev/disk/by-uuid/46E1-B3BA

UDEV  [1191447365.598111] add      /block/sdb/sdb1 (block)
UDEV_LOG=3
ACTION=add
DEVPATH=/block/sdb/sdb1
SUBSYSTEM=block
SEQNUM=2828
MINOR=17
MAJOR=8
PHYSDEVPATH=/devices/pci0000:00/0000:00:13.2/usb1/1-6/1-6:1.0/host6/target6:0:0/6:0:0:0
PHYSDEVBUS=scsi
PHYSDEVDRIVER=sd
UDEVD_EVENT=1
DEVTYPE=partition
ID_VENDOR=Kingston
ID_MODEL=DataTraveler_2.0
ID_REVISION=1.00
ID_SERIAL=Kingston_DataTraveler_2.0_0000015955-0:0
ID_SERIAL_SHORT=0000015955
ID_TYPE=disk
ID_INSTANCE=0:0
ID_BUS=usb
ID_PATH=pci-0000:00:13.2-usb-0:6:1.0-scsi-0:0:0:0
ID_FS_USAGE=filesystem
ID_FS_TYPE=ext3
ID_FS_VERSION=1.0
ID_FS_UUID=9c19c221-e338-49a7-9614-df76a4b940e2
ID_FS_UUID_ENC=9c19c221-e338-49a7-9614-df76a4b940e2
ID_FS_LABEL=Xubuntu
ID_FS_LABEL_ENC=Xubuntu
ID_FS_LABEL_SAFE=Xubuntu
DEVNAME=/dev/sdb1
DEVLINKS=/dev/disk/by-id/usb-Kingston_DataTraveler_2.0_0000015955-0:0-part1 /dev/disk/by-path/pci-0000:00:13.2-usb-0:6:1.0-scsi-0:0:0:0-part1 /dev/disk/by-uuid/9c19c221-e338-49a7-9614-df76a4b940e2 /dev/disk/by-label/Xubuntu

Also, dbus-monitor seems to be sending out signals to Gnome to mount the two new partitions:

signal sender=:1.4 -> dest=(null destination) path=/org/gnome/GnomeVFS/Daemon; interface=org.gnome.GnomeVFS.Daemon; member=DriveConnectedSignal
   struct {
      int32 8
      int32 3
      array [
      ]
      string "/dev/sdb2"
      string ""
      string "Kingston DataTraveler 2.0"
      string "gnome-dev-removable-usb"
      string "/org/freedesktop/Hal/devices/volume_uuid_46E1_B3BA"
      boolean true
      boolean true
      boolean false
   }
signal sender=:1.4 -> dest=(null destination) path=/org/gnome/GnomeVFS/Daemon; interface=org.gnome.GnomeVFS.Daemon; member=DriveConnectedSignal
   struct {
      int32 9
      int32 3
      array [
      ]
      string "/dev/sdb1"
      string ""
      string "Kingston DataTraveler 2.0 (2)"
      string "gnome-dev-removable-usb"
      string "/org/freedesktop/Hal/devices/volume_uuid_9c19c221_e338_49a7_9614_df76a4b940e2"
      boolean true
      boolean true
      boolean false
   }

Furthermore, vol_id AND blkid both correctly report the newly inserted volumes.

On my system at least, the problem seems to be with gnome-volume-manager, which reports the following:

$ gnome-volume-manager -n
# wait a few seconds...
# insert flash drive
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/usb_device_951_1603_0000015955
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/usb_device_951_1603_0000015955_if0
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/usb_device_951_1603_0000015955_usbraw
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/usb_device_951_1603_0000015955_if0_scsi_host
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/usb_device_951_1603_0000015955_if0_scsi_host_scsi_device_lun0
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/usb_device_951_1603_0000015955_if0_scsi_host_scsi_device_lun0_scsi_generic
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/storage_serial_Kingston_DataTraveler_2_0_0000015955_0_0
manager.c/2303: not a mountable volume: /org/freedesktop/Hal/devices/storage_serial_Kingston_DataTraveler_2_0_0000015955_0_0
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/volume_uuid_46E1_B3BA
manager.c/2349: Changed: /dev/sdb2
manager.c/3306: gvm_user_active_at_console: check-foreground-console returned with 0
manager.c/2538: Device added: /org/freedesktop/Hal/devices/volume_uuid_9c19c221_e338_49a7_9614_df76a4b940e2
manager.c/2349: Changed: /dev/sdb1


What does "not a mountable volume mean"?  Presumably, that's the key to solving this bug.  Let me know if I can provide any other helpful information.