I have been doing some searching and experimenting with the same idea. I need it for templates of counter tops. But I don't have a cnc router yet so I didn't spend a lot of time on it. But this is what I came up with so far. Build a frame like the one
HERE
Use rotary encoders with rubber wheels moving along a smooth surface. Then have a PIC reading the encoders and just keeping track of current position. When you need a point entered in to cad just press a button and the PIC will send a Keyboad style serial signal to the cad software with the apropriate charachters to plot the point. Last thing is get the scale right and it should be a nice cheap digitizer. Only thing is im not a programming wiz so it will take me a while to get this thing going. If someone would like to trade PIC programming skills for cnc related goodies or other similar things let me know.
There is another way to do it, there is software called DRO I posted it in another thread. It works in DOS and reads encoders and displays the position on the screen. What you can do is use it and type the nubers in to cad manualy you will still have the same results but it will take more time.