Cms over on the EeeUser Forums has a review / user guide on the PNF Duo Tablet Pen for laptops. It’s not readily available in certain countries but he bought his from Ebay for about $70. Manuals were in English.
The Duo Tablet Pen allows you to use any suface as a drawing pad, or as a more common use, transforming your display into a touchscreen. It uses a combination of ultrasonic sounds and infrared light to make the magic happen.
Overall he seems quite pleased with the Duo Tablet Pen on his ASUS Eee PC 1000HE saying it’s “a gadget that I never knew I needed or knew existed until now” and it replaces the mouse on trips outside.

The downsides? Apart from the obvious bulk and wires added to the netbook, no pressure sensitivity and no operation by finger or stylus. You have to use the pen provided. I would also use a protective film on the display to prevent scratches.
Check out the Duo Laptop Pen Guide / Review. You can visit PNF for more information about the product.
