If I am reading the KEY bitmask correctly, the blacklist in drivers/input/joydev.c is not matching and excluding the device from being presented as a joystick, because the tablet input device does not claim BTN_TOUCH in its keybits. (BTN_TOUCH = 0x14a, which is a mask of 400 0 0 0 0 0, which doesn't correspond to any of the bits set in the above keybits.)
Looking at the upstream kernel sources for 2.6.27, it looks like BTN_TOUCH *is* be getting set for wacom tablet keybits in drivers/input/tablet/wacom_sys.c, so this may be something broken specifically in Ubuntu.
It's definitely a kernel issue; /proc/bus/ input/devices reports:
I: Bus=0003 Vendor=056a Product=0042 Version=0126 devices/ pci0000: 00/0000: 00:1d.0/ usb4/4- 1/4-1:1. 0/input/ input19
N: Name="Wacom Intuos2 6x8"
P: Phys=
S: Sysfs=/
U: Uniq=
H: Handlers=mouse1 event7 js0
B: EV=1f
B: KEY=18df 1f0000 0 0 0 0
B: REL=100
B: ABS=1000f000163
B: MSC=1
If I am reading the KEY bitmask correctly, the blacklist in drivers/ input/joydev. c is not matching and excluding the device from being presented as a joystick, because the tablet input device does not claim BTN_TOUCH in its keybits. (BTN_TOUCH = 0x14a, which is a mask of 400 0 0 0 0 0, which doesn't correspond to any of the bits set in the above keybits.)
Looking at the upstream kernel sources for 2.6.27, it looks like BTN_TOUCH *is* be getting set for wacom tablet keybits in drivers/ input/tablet/ wacom_sys. c, so this may be something broken specifically in Ubuntu.