My A818 is already supported as a MTP device in Hardy. Just try to plug it in and launch Rhythmbox.

However, I would feel very angry to see that Ubuntu used to support it as a mass-storage device, but would prefer to use a proprietary drm-friendly protocol with the following drawbacks :
- it would not be possible to feed the device with a simple drag & drop
- it would be mandatory to use a special software (e.g. supporting MTP, like Rhythmbox) to load music on the player
- it would be impossible to load videos nor photos on the device, as Rhythmbox does not support video or photographic content

The only question to ask is : what has changed in HAL fdi rules (or in whatever underlying layer located between the kernel and HAL) between Gutsy and Hardy that just prevent HAL from exposing the device's FAT32 volume to other programs.

gnome-mount - doesn't see the volume because it is not exposed by HAL
HAL - detects the device, probe for a volume, find it, but does not create volume information in his tree (see following debug log)
Kernel - see the volume and is able to manually mount it using mount or pmount

Apr  8 14:11:39 thinkpad hald-addon-storage: [13675]: 14:11:39.102 [D] addon-storage.c:748: ************************************************** 
Apr  8 14:11:39 thinkpad hald-addon-storage: [13675]: 14:11:39.102 [D] addon-storage.c:749: Doing addon-storage for /dev/sdb (bus usb) (drive_type disk) (udi /org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979) 
Apr  8 14:11:39 thinkpad hald-addon-storage: [13675]: 14:11:39.102 [D] addon-storage.c:750: ************************************************** 
Apr  8 14:11:41 thinkpad orage: polling /dev/sdb (every 2 sec): [13675]: 14:11:41.003 [I] addon-storage.c:355: Checking whether device /dev/sdb is locked on HAL 
Apr  8 14:11:41 thinkpad orage: polling /dev/scd0 (every 2 sec): [5329]: 14:11:41.003 [I] addon-storage.c:355: Checking whether device /dev/scd0 is locked on HAL 
Apr  8 14:11:41 thinkpad orage: polling /dev/scd0 (every 2 sec): [5329]: 14:11:41.004 [I] addon-storage.c:363: ... device /dev/scd0 is not locked on HAL 
Apr  8 14:11:41 thinkpad orage: polling /dev/sdb (every 2 sec): [13675]: 14:11:41.004 [I] addon-storage.c:363: ... device /dev/sdb is not locked on HAL 
Apr  8 14:11:41 thinkpad orage: polling /dev/sdb (every 2 sec): [13675]: 14:11:41.006 [D] addon-storage.c:542: Media insertion detected on /dev/sdb 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.010 [D] probe-storage.c:155: Doing probe-storage for /dev/sdb (bus usb) (drive_type disk) (udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979) (--only-check-for-fs==1) 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.010 [D] probe-storage.c:407: Checking for file system on /dev/sdb 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.010 [D] probe-storage.c:413: Doing open ("/dev/sdb", O_RDONLY) 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.012 [D] probe-storage.c:421: Returned from open(2) 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.012 [D] probe-storage.c:437: look for existing partitions for sdb 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.012 [D] probe-storage.c:447: partition sdb1 found, skip probing for filesystem 
Apr  8 14:11:41 thinkpad hald-probe-storage: [13682]: 14:11:41.016 [I] partutil.c:875: MSDOS partition table detected