What is the yellow wire used for? Allow me to quote "Hacking the XBox" by Bunnie Huang:
"The video sync signal is a 3.3V CMOS or TTL-compatible signal. It is a basic 15.734 kHz positive polarity pulse train synchronized to the horizontal line time of the composite video output, with a single longer pulse at the beginning of every video field. This signal enable peripherals that are pointed at the TV screen, such as a light pen or a light gun for shooting games, to derive position information."
What does this mean? A device that as of yet does not exist would use this signal combined with a lens & photodetector to determine where at the screen you are pointing. I doubt that it will ever be utilized at this point, unless the Linux guys write something to use it.