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I'm assuming it would appear as a generic HID. It should work the same as a mouse, really, except that it will move instantly between points so events like mouse over won't happen unless the pen is touching the surface.
It really depends on the device, though. Get the device list in your OS of choice and see what kind of driver is controlling it.
i think this is a tough topic cause tablet pc is still not so popular in the market
i tried to purchase one but quite slow: compaq tc1000
hopefully the pen share the same io ports like mouse
You will need to implement a usb stack and than add support for HID devices.
Alot of work, no hobby OS that i know of as such support.
Here a starting point, go here: http://ticon.net/~tatimmer/linux.html
and get DDUSB.