Ten years ago I bought a wacom penpartner USB.
I added some lines for it to be included with the wacom project.
It does get picked up by the kernel now, but it doesn't always work out of the box.

For ubuntu / xubuntu find/add the following lines to /etc/X11/xorg.conf :

Section "InputDevice"
	Driver "wacom"
	Identifier "stylus"
	Option "Device" "/dev/input/tablet-penpartner"
	Option "Type" "stylus"
	Option "Mode" "Absolute"
	Option "USB" "on"
EndSection

Section "ServerLayout"
	Identifier	"Default Layout"
	InputDevice	"stylus"	"SendCoreEvents"
  screen "Default Screen"
EndSection
in a terminal type: 
xsetwacom set stylus button2 3
this maps the rightmousebutton on the stylus(pen) to the rightmousebutton instead of the middlemousebutton

Update march 2009 xubuntu 9.04:

Something changed again, newer versions of X don't seem to use xorg.conf anymore
To change the sidebutton of the stylus(pen)
to the rightmouse button type in a shell: 

xmodmap -e "pointer = 1 3 2"
to do this everytime at boottime create a file called .Xmodmap in your home directory (~) with: 
pointer = 1 3 2
in it...