The original XO-1 came equipped with a unique dual-mode pointing device. It functioned both as a conventional trackpad (the capactive-sensitive "Glide Sensor") and as a graphics tablet (the pressure-sensitive "Pen Tablet"). Starting in early 2009 this dual-mode pad is replaced by a regular trackpad without tablet support .
Basic information about how the touchpad is intended to be used can be found in the Human Interface Guidelines.
The hardware was manufactured by ALPS Electric. The specification (File:KGDMFA001-non-confidential.pdf) may require Acrobat reader with Japanese font package also installed for successful viewing and printing). See http://lists.laptop.org/pipermail/devel/2007-August/006080.html for missing details.
In Terminal activity enter
dmesg | grep psmouse
Instructions from a mailing list post.
In Terminal Activity, to disable the touchpad, enter the command
xsetpointer -c "OLPC ALPS HGPK"
To enable the touchpad, enter the command
xsetpointer +c "OLPC ALPS HGPK"
You don't need to be root in either case.
PenTablet Software Support
In 2.6.31 based kernels, the touchpad can be put into pentablet mode.
The UI consists of:
echo [1|0] > /sys/devices/platform/i8042/serio1/ptmode
to turn pen tablet mode on or off. You can also add a a file to the /etc/modprobe.d directory containing "options psmouse ptmode=1" to force that mode at boot time.
Note that this does _not_ give the laptop a true tablet device. What these changes basically let you do is select between two different mouse-like touchpads, i.e., a) the normal (capacitive) touchpad, and b) a much wider (resistive) touchpad that behaves very similarly except that you have to press pretty hard with stylus (or maybe a fingernail) to get it to do anything.
It may also be possible to get pentablet support via this older userspace trick: use the resistive touchpad from userspace.