Just got my Wacom tablet working under Xfree86 4.0.1... pretty happy about that, especially since the documentation that comes with it doesn't seem to give you everything you might need.

The extract that I use is below:

Section "InputDevice"
        Identifier      "Generic Stylus"
        Driver          "wacom"
        Option          "Device"        "/dev/ttyS1"
        Option          "Type"          "stylus"
        Option          "Mode"          "absolute"
        Option          "AlwaysCore"
EndSection
Section "InputDevice"
        Identifier      "Generic Eraser"
        Driver          "wacom"
        Option          "Device"        "/dev/ttyS1"
        Option          "Type"          "eraser"
        Option          "Mode"          "absolute"
        Option          "AlwaysCore"
EndSection

Then you just add the two to the ServerLayout section like so:

Section "ServerLayout"
        Identifier      "Default Layout"
        Screen          "Default Screen"
        InputDevice     "Generic Keyboard"
        InputDevice     "Generic Mouse"
        InputDevice     "Generic Stylus"
        InputDevice     "Generic Eraser"
EndSection

And voila!