I have one of those IBM UltraNav keyboards which features the "nipple" input device as well as a Synaptics touchpad.

Unfortunately, the touchpad gets in the way of my regular typing. It turns out that it is possible to disable the touchpad (and keep the nipple active). This technique is a little bit evil, but it's all I've come up with so far.

You have to echo a magic string into the "unbind" device present within the usbhid driver's sysfs entry. This works for me: 

echo -n 2-1.4\:1.0 > /sys/bus/usb/drivers/usbhid/unbind