Comment 13 for bug 522318

@Tom Jaeger
Bingo! Your /etc/udev/rules.d/65-xorg-wacom.rules file did the trick! I seriously wonder, though, why it hadn't worked before. The situation was that I had added the udev rules directly into /lib/udev/rules.d/69-xserver-xorg-input-wacom.rules.

When having it configured that way 'udevadm info --path=/sys/class/tty/ttyS0/ --attribute-walk' gives

  looking at device '/devices/pnp0/00:08/tty/ttyS0':
    KERNEL=="ttyS0"
    SUBSYSTEM=="tty"
    DRIVER==""

  looking at parent device '/devices/pnp0/00:08':
    KERNELS=="00:08"
    SUBSYSTEMS=="pnp"
    DRIVERS=="serial"
    ATTRS{id}=="WACf004"

  looking at parent device '/devices/pnp0':
    KERNELS=="pnp0"
    SUBSYSTEMS==""
    DRIVERS==""

After cleaning up /lib/udev/rules.d/69-xserver-xorg-input-wacom.rules to its original state and installing /etc/udev/rules.d/65-xorg-wacom.rules from comment #12 the digitizer appears to be fully working. The mouse cursor follows the digitizer. Mouseclicks work with both ends of the pen. A brief test with xournal shows pressure sensitivity working as well as the button on the pen. This is great. Thanks for the effort.

In case this is helpful to anyone -- this is how the logs look on my Motion Computing LE1700, now that it's working:

/var/log/udev:
(...)
UDEV [1267608189.360156] add /devices/pnp0/00:08 (pnp)
UDEV_LOG=3
ACTION=add
DEVPATH=/devices/pnp0/00:08
SUBSYSTEM=pnp
DRIVER=serial
SEQNUM=1316
NAME=Serial Wacom Tablet
ID_INPUT=1
ID_INPUT_TABLET=1
x11_driver=wacom
(...)

/var/log/Xorg.0.log:
(...)
(II) config/udev: Adding input device Serial Wacom Tablet (/dev/ttyS0)
(II) LoadModule: "wacom"
(II) Loading /usr/lib/xorg/modules/input/wacom_drv.so
(II) Module wacom: vendor="X.Org Foundation"
        compiled for 1.7.3.902, module version = 0.10.3
        Module class: X.Org XInput Driver
        ABI class: X.Org XInput driver, version 7.0
(**) Option "Device" "/dev/ttyS0"
(II) Serial Wacom Tablet: type not specified, assuming 'stylus'.
(II) Serial Wacom Tablet: other types will be automatically added.
(**) Serial Wacom Tablet: always reports core events
(II) Serial Wacom Tablet: hotplugging dependent devices.
(**) Option "Device" "/dev/ttyS0"
(**) Serial Wacom Tablet eraser: always reports core events
(II) XINPUT: Adding extended input device "Serial Wacom Tablet eraser" (type: ERASER)
(**) Option "StopBits" "1"
(**) Option "DataBits" "8"
(**) Option "Parity" "None"
(**) Option "Vmin" "1"
(**) Option "Vtime" "10"
(**) Option "FlowControl" "Xoff"
(WW) Serial Wacom Tablet eraser: unable to read ISDV4 % data after 3 tries at (38400)
(--) Serial Wacom Tablet eraser: using pressure threshold of 15 for button 1
(--) Serial Wacom Tablet eraser: Wacom General ISDV4 tablet speed=38400 maxX=24576 maxY=18432 maxZ=255 resX=2540 resY=2540 tilt=disabled
(--) Serial Wacom Tablet eraser: top X=0 top Y=0 bottom X=24576 bottom Y=18432 resol X=2540 resol Y=2540
(II) Serial Wacom Tablet: hotplugging completed.
(II) XINPUT: Adding extended input device "Serial Wacom Tablet" (type: STYLUS)
(--) Serial Wacom Tablet: top X=0 top Y=0 bottom X=24576 bottom Y=18432 resol X=2540 resol Y=2540
(...)