Useful for mods. Take out the phone jack, solder in a 4 pin S-Video port and wire up 5v and ground from modem board plus ADB line from Duo's EXT connector and you have a built in ADB port. No need for a mini dock to use a different keyboard or mouse or use ADB peripheral.
I've done it. The hardest part is soldering the wire to the right pin of the Duo expansion connector. Here's a still-working page that shows how to wire up ADB port: http://home.comcast.net/~gionpeters/hhhh/duo.htm