Everyone, Team 862 has solved your problem:
Playstation 2 Adapter
Link is to the discussion and white paper for our adapter. Build the circuit, download the code into the PIC, plug in your off-the-shelf PS2 controller and away you go!
It is competition legal, our 2006 robot is driven off these. One for driver and one for co-pilot.
Winner of the "Delphi Driving Tomorrow's Technology" award at the 2006 Milwaukee Regional.