This idea takes the tablet mac concept to the next level. Tablet PCs don't usea touchscreen, they use a real Wacom tablet behind the LCD. The challange is that the external tablets are either too small or too large to fit iniside the laptop case. Check out this link to wacom's site:

These are the INTERNAL tablets, the controller section has details on the connections, its basically a serial port. While the site does not offer units for sale our good pal ebay does. However connecting the serial controller is a little out of my league. This is the product we need to make this a reality.
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...